View source for Tunnel

{{short description|Underground passage made for traffic}}
{{about|underground passages}}
{{Use dmy dates|date=August 2023}}
[[File:Signalisation routière entrée sud tunnel du Galibier.jpg|thumb|right|Tunnel in [[Col du Galibier]], France]]
[[File:14-46-35-f-mutzig.jpg|thumb|right|Tunnel in [[Fort de Mutzig]], France]]
[[File:Tunel en Guanajuato.jpg|thumb|Decorated portal to a road tunnel in [[Guanajuato]], Mexico]]
[[File:Heatpipe tunnel copenhagen 2009.jpg|thumb|[[Utility tunnel]] for heating pipes between [[Rigshospitalet]] and Amagerværket in [[Copenhagen]], Denmark]]
[[File:A crossover on the south side of Zhongxiao Xinsheng Station.JPG|thumb|Tunnel on the [[Taipei Metro]] in [[Taiwan]]]]
[[File:Southern portal of Chirk canal tunnel - geograph.org.uk - 1293092.jpg|thumb|Southern portal of the {{convert|421|m|ft|abbr=on|adj=mid|-long}} [[Chirk Tunnel|Chirk canal tunnel]], Wales]]

A '''tunnel''' is an underground or undersea passageway. It is dug through surrounding soil, earth or rock, or laid under water, and is usually completely enclosed except for the two [[Portal (architecture)|portals]] common at each end, though there may be access and ventilation openings at various points along the length. A [[Pipeline transport|pipeline]] differs significantly from a tunnel,<ref>{{Cite journal |last1=Zhou |first1=Man |last2=Su |first2=Xiaolong |last3=Chen |first3=Yaying |last4=An |first4=Lin |date=2022-10-02 |title=New Technologies and Challenges in the Construction of the Immersed Tube Tunnel of the Hong Kong-Zhuhai-Macao Link |url=https://www.tandfonline.com/doi/full/10.1080/10168664.2021.1904487 |journal=Structural Engineering International |language=en |volume=32 |issue=4 |pages=455–464 |doi=10.1080/10168664.2021.1904487 |issn=1016-8664|url-access=subscription }}</ref>{{Clarify|date=March 2025}} though some recent tunnels have used [[immersed tube]] construction techniques rather than traditional tunnel boring methods.<ref name=":1">{{Cite book |last=Takahashi |first=Yutaka |title=Water Storage, Transport, and Distribution |publisher=[[EOLSS Publications]] |year=2009 |isbn=9781848261761 |pages=318–319 |language=EN}}</ref>

A tunnel may be for foot or vehicular [[road]] [[traffic]], for [[rail transport|rail]] traffic, or for a [[canal]]. The central portions of a [[rapid transit]] network are usually in the tunnel. Some tunnels are used as [[sanitary sewer|sewers]] or [[aqueduct (watercourse)|aqueducts]] to supply water for consumption or for [[hydroelectric]] stations. [[Utility tunnel]]s are used for routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment.<ref name=":0">Salazar, Waneta. ''Tunnels in Civil Engineering''. Delhi, India : White Word Publications, 2012.</ref>

[[Secret passage|Secret tunnels]] are built for military purposes, or by civilians for [[smuggling]] of [[weapon]]s, [[contraband]], or [[people]].<ref>{{Cite journal |last=Sorrensen |first=Cynthia |date=2014-07-01 |title=Making the Subterranean Visible: Security, Tunnels, and the United States–Mexico Border |url=https://www.tandfonline.com/doi/full/10.1111/j.1931-0846.2014.12029.x |journal=Geographical Review |language=en |volume=104 |issue=3 |pages=328–345 |doi=10.1111/j.1931-0846.2014.12029.x |bibcode=2014GeoRv.104..328S |issn=0016-7428|url-access=subscription }}</ref> Special tunnels, such as [[wildlife crossing]]s, are built to allow wildlife to cross human-made barriers safely.<ref>Brodziewska, J. (2005). Wildlife tunnels and fauna bridges in Poland: past, present and future, 1997-2013. ''UC Davis: Road Ecology Center''. Retrieved from <nowiki>https://escholarship.org/uc/item/4wd0j27j</nowiki></ref> Tunnels can be connected together in [[tunnel network]]s.

A tunnel is relatively long and narrow; the length is often much greater than twice the [[diameter]], although similar shorter excavations can be constructed, such as cross passages between tunnels. The definition of what constitutes a tunnel can vary widely from source to source. For example, in the United Kingdom, a road tunnel is defined as "a subsurface highway structure enclosed for a length of {{convert|150|m|ft}} or more."<ref>{{cite book |url=https://www.standardsforhighways.co.uk/tses/attachments/987a669b-13a1-40b9-94da-1ea4e4604fdd |title=Highway Structures & Bridges Design CD 352, Design of road tunnels (formerly BD 78/99) |publisher=The Department for Transport |year=2020}}</ref> In the United States, the [[National Fire Protection Association|NFPA]] definition of a tunnel is "An underground structure with a design length greater than {{convert|23|m|ft|abbr=on}} and a diameter greater than {{convert|1800|mm|ft}}."<ref>{{cite book |title=NFPA Standard for Safeguarding Construction, Alteration, and Demolition Operations |publisher=National Fire Protection Association}}</ref>

== Etymology ==
[[File:Inside the Salang Tunnel in November 2013.jpg|thumb|Salang Tunnel in Afghanistan.]]
The word "tunnel" comes from the [[Middle English]] ''tonnelle'', meaning "a net", derived from [[Old French]] ''tonnel'', a diminutive of ''tonne'' ("cask"). The modern meaning, referring to an underground passageway, evolved in the 16th century as a metaphor for a narrow, confined space like the inside of a cask.<ref>{{Citation |title=tunnel, n. |date=2023-03-02 |work=Oxford English Dictionary |url=https://oed.com/dictionary/tunnel_n |access-date=2024-09-01 |edition=3 |publisher=Oxford University Press |language=en |doi=10.1093/oed/1137765320|url-access=subscription }}</ref><ref>{{Cite AV media |url=https://www.imdb.com/title/tt4175506/plotsummary/ |title="Mahabharat" Escape from Lakshagraha (TV Episode 1989) - Plot |publisher=IMDb |language=en-US |access-date=2024-12-07}}</ref><ref name="Sengar">{{Cite news |last=Sengar |first=Resham |title=Remember the Lakh Ka Ghar of Mahabharata? ASI will begin excavating the site in Dec |url=https://timesofindia.indiatimes.com/travel/destinations/remember-the-lakh-ka-ghar-of-mahabharata-asi-will-begin-excavating-the-site-in-dec/articleshow/61487314.cms |access-date=2024-12-07 |work=The Times of India |issn=0971-8257}}</ref>

== History ==
{{see also|History of water supply and sanitation}}
It's more likely that the first tunneling was done made by [[prehistoric people]] seeking to enlarge their [[Cave|caves.]]<ref name=":1" />

[[Babylon]], about 2200 B.C., it is believed that the first artificial tunnel was constructed. To join the temple of Belos with the palace, this was built with the aid of the [[Tunnel construction#Cut-and-cover|cut and cover]] technique.<ref>{{Cite web |title=Ancient Period - About Tunnelling - ITA-AITES |url=https://tunnel.ita-aites.org/en/cases-histories/history/ancient-period |access-date=2024-12-07 |website=tunnel.ita-aites.org}}</ref>

Some of the earliest tunnels used by humans were [[paleoburrow]]s excavated by prehistoric mammals.<ref>{{Cite journal |last=Frank, de Carvalho Buchmann, Gonçalves de Lima, Caron, Lopes & Fornari |date=2011 |title=KARSTIC FEATURES GENERATED FROM LARGE PALAEOVERTEBRATE TUNNELS IN SOUTHERN BRAZIL |url=https://www.ufrgs.br/paleotocas/Frank_et_al_2011b.pdf |journal=Espeleo-Tema |volume=22 |issue=1}}</ref>[[File:Joralemon Street Tunnel postcard, 1913.jpg|thumb|Joralemon Street Tunnel on 1913 postcard, part of the [[New York City Subway]] system]]

Much of the early technology of tunnelling evolved from [[mining]] and [[military engineering]]. The etymology of the terms "mining" (for mineral extraction or for [[mining (military)|siege attacks]]), "[[military engineering]]", and "[[civil engineering]]" reveals these deep historic connections.

=== Antiquity and early middle ages ===
Predecessors of modern tunnels were [[adit]]s that transported water for [[irrigation]], drinking, or [[sewerage]]. The first [[qanat]]s are known from before 2000 BC.

The earliest tunnel known to have been excavated from both ends is the [[Siloam tunnel|Siloam Tunnel]], built in [[Jerusalem]] by the kings of [[Kingdom of Judah|Judah]] around the 8th century BC.<ref>{{Cite web |last=gzichnalis |date=2022-01-15 |title=Eupalinos Tunnel – First Tunnel to be excavated simultaneously from both ends in the 6th centurty B.C. |url=https://wtc2023.gr/eupalinos-tunnel-first-tunnel-to-be-excavated-simultaneously-from-both-ends-in-the-6th-centurty-b-c/ |access-date=2024-09-26 |website=WTC2023 |language=en-US}}</ref> Another tunnel excavated from both ends, maybe the second known, is the [[Tunnel of Eupalinos]], which is a tunnel [[Aqueduct (water supply)|aqueduct]] {{convert|1036|m|ft|abbr=on|adj=on|sigfig=3}} long running through Mount Kastro in [[Samos Island|Samos]], Greece. It was built in the 6th century BC to serve as an [[Aqueduct (water supply)|aqueduct]].

In [[Pakistan]], the mughal era tunnel has been restored in the [[Lahore]].<ref>{{Cite web |last=Mehmood |first=Asif |date=2021-03-18 |title=400-year-old tunnel discovered in Shahi Qila |url=https://tribune.com.pk/story/2290002/400-year-old-tunnel-discovered-in-shahi-qila |access-date=2024-12-16 |website=The Express Tribune |language=en}}</ref><ref>{{Cite web |last=Omer |first=Shahab |date=2018-10-09 |title=Mughal-era tunnel, underground chambers discovered at Lahore Fort {{!}} Pakistan Today |url=https://archive.pakistantoday.com.pk/2018/10/09/mughal-era-tunnel-underground-chambers-discovered-at-lahore-fort/#:~:text=LAHORE:%20A%20tunnel,%20structures%20with,age%20to%20the%20Mughal%20Era. |access-date=2024-12-16 |language=en-GB}}</ref>

In [[Ethiopia]], the [[Siqurto foot tunnel]], hand-hewn in the Middle Ages, crosses a mountain ridge.

In the [[Gaza Strip]], the network of tunnels was used by Jewish strategists as rock-cut shelters, in first links to Judean resistance against Roman rule in the [[Bar Kokhba revolt]] during the 2nd century AD.

== Geotechnical investigation and design ==
{{Main|Geotechnical investigation}}
A major tunnel project must start with a comprehensive investigation of ground conditions by collecting samples from [[borehole]]s and by other geophysical techniques.<ref>{{Cite web| title=Guide for tunnelling work | url=https://www.safeworkaustralia.gov.au/system/files/documents/1702/guide-tunnelling.pdf | archive-url=https://web.archive.org/web/20180408083316/https://www.safeworkaustralia.gov.au/system/files/documents/1702/guide-tunnelling.pdf | archive-date=2018-04-08}}</ref> An informed choice can then be made of machinery and methods for excavation and ground support, which will reduce the risk of encountering unforeseen ground conditions. In planning the route, the horizontal and vertical alignments can be selected to make use of the best ground and water conditions. It is common practice to locate a tunnel deeper than otherwise would be required, in order to excavate through solid rock or other material that is easier to support during construction.

Conventional desk and preliminary site studies may yield insufficient information to assess such factors as the blocky nature of rocks, the exact location of fault zones, or the stand-up times of softer ground. This may be a particular concern in large-diameter tunnels. To give more information, a pilot tunnel (or "drift tunnel") may be driven ahead of the main excavation. This smaller tunnel is less likely to collapse catastrophically should unexpected conditions be met, and it can be incorporated into the final tunnel or used as a backup or emergency escape passage. Alternatively, horizontal boreholes may sometimes be drilled ahead of the advancing tunnel face.

Other key geotechnical factors:
* '''Stand-up time''' is the amount of time a newly excavated cavity can support itself without any added structures. Knowing this parameter allows the engineers to determine how far an excavation can proceed before support is needed, which in turn affects the speed, efficiency, and cost of construction. Generally, certain configurations of rock and clay will have the greatest stand-up time, while sand and fine soils will have a much lower stand-up time.<ref>{{cite book |editor1-last= Bickel|editor1-first= John O.|editor2-first =Thomas R.|editor2-last = Kuesel |editor3-first=Elwyn H.|editor3-last =King|page = 210|date = 2004|title= Tunnel Engineering Handbook|edition = 2nd|publisher =Kluwer Academic Publishers|chapter = Tunnel Boring Machines|first = Harry|last = Sutcliffe|chapter-url = https://books.google.com/books?id=59V5BgAAQBAJ&pg=PA210|isbn = 978-1-4613-8053-5}}</ref>
* [[Groundwater]] control is very important in tunnel construction. Water leaking into a tunnel or vertical shaft will greatly decrease stand-up time, causing the excavation to become unstable and risking collapse. The most common way to control groundwater is to install dewatering pipes into the ground and to simply pump the water out.<ref>Powers, P.J. (2007). Construction dewatering and groundwater control. Hoboken, NJ: John Wiley & Sons Inc.</ref> A very effective but expensive technology is [[ground freezing]], using pipes which are inserted into the ground surrounding the excavation, which are then cooled with special refrigerant fluids. This freezes the ground around each pipe until the whole space is surrounded with frozen soil, keeping water out until a permanent structure can be built.
* Tunnel [[Cross section (geometry)|cross-sectional shape]] is also very important in determining stand-up time. If a tunnel excavation is wider than it is high, it will have a harder time supporting itself, decreasing its stand-up time. A square or rectangular excavation is more difficult to make self-supporting, because of a concentration of [[Stress (mechanics)|stress]] at the corners.<ref name="Engineers. 1978"/>

=== Choice of tunnels versus bridges ===
{{unreferenced section|date=February 2021}}
{{Anchor|Choice of tunnels vs bridges}}
[[File:2007 09 19 - 895tunnel - WB 3.JPG|right|thumb|The [[Harbor Tunnel (Baltimore)|Harbor Tunnel]] in [[Baltimore]], USA, which carries [[Interstate 895 (Maryland)|I-895]], serves as an example of a water-crossing tunnel built instead of a bridge.]]
For water crossings, a tunnel is generally more costly to construct than a bridge.<ref>{{Cite web |title=Tunnels - an environmentally attractive option? |url=https://www.worldhighways.com/wh10/feature/tunnels-environmentally-attractive-option |access-date=2024-10-08 |website=World Highways |language=en}}</ref> However, both navigational and traffic considerations may limit the use of high bridges or [[drawbridge]]s intersecting with shipping channels, necessitating a tunnel.

Bridges usually require a larger footprint on each shore than tunnels. In areas with expensive real estate, such as [[Manhattan]] and urban [[Hong Kong]], this is a strong factor in favor of a tunnel. Boston's [[Big Dig]] project replaced elevated roadways with a tunnel system to increase traffic capacity, hide traffic, reclaim land, redecorate, and reunite the city with the waterfront.<ref>{{Cite web |title=FHWA - Center for Innovative Finance Support - Project Profiles |url=https://www.fhwa.dot.gov/ipd/project_profiles/ma_boston_central_artery.aspx |access-date=2024-11-08 |website=www.fhwa.dot.gov}}</ref>

The 1934 [[Queensway Tunnel]] under the [[River Mersey]] at [[Liverpool]] was chosen over a massively high bridge partly for defence reasons; it was feared that aircraft could destroy a bridge in times of war, not merely impairing road traffic but blocking the river to navigation.<ref>{{Cite web |title=Queensway Tunnel which links Liverpool to Birkenhead marks 90th anniversary |url=https://www.bbc.com/news/articles/c7283vplp56o |access-date=2024-10-08 |website=www.bbc.com |date=20 July 2024 |language=en-GB}}</ref> Maintenance costs of a massive bridge to allow the world's largest ships to navigate under were considered higher than for a tunnel. Similar conclusions were reached for the 1971 [[Kingsway Tunnel]] under the Mersey. In [[Hampton Roads, Virginia]], tunnels were chosen over bridges for strategic considerations; in the event of damage, bridges might prevent [[US Navy]] vessels from leaving [[Naval Station Norfolk]].

Water-crossing tunnels built instead of bridges include the [[Seikan Tunnel]] in Japan; the [[Holland Tunnel]] and [[Lincoln Tunnel]] between [[New Jersey]] and Manhattan in [[New York City]]; the [[Queens-Midtown Tunnel]] between Manhattan and the [[Borough (New York City)|borough]] of [[Queens]] on [[Long Island]]; the [[Detroit-Windsor Tunnel]] between [[Michigan]] and [[Ontario, Canada|Ontario]]; and the [[Elizabeth River (Virginia)|Elizabeth River]] tunnels between [[Norfolk, Virginia|Norfolk]] and [[Portsmouth, Virginia]]; the 1934 [[River Mersey]] road [[Queensway Tunnel]]; the [[Western Scheldt Tunnel]], Zeeland, Netherlands; and the [[North Shore Connector]] tunnel in [[Pittsburgh, Pennsylvania]]. The [[Sydney Harbour Tunnel]] was constructed to provide a second harbour crossing and to alleviate traffic congestion on the [[Sydney Harbour Bridge]], without spoiling the iconic view.

Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather, and shipping during construction (as in the {{convert|51.5|km|mi|adj=on|disp=or}} [[Channel Tunnel]]), aesthetic reasons (preserving the above-ground view, landscape, and scenery), and also for weight capacity reasons (it may be more feasible to build a tunnel than a sufficiently strong bridge).

Some water crossings are a mixture of bridges and tunnels, such as the [[Oresund Bridge|Denmark to Sweden link]] and the [[Chesapeake Bay Bridge-Tunnel]] in [[Virginia]].

There are particular hazards with tunnels, especially from vehicle fires when combustion gases can [[asphyxia]]te users, as happened at the [[Gotthard Road Tunnel#2001 collision and fire|Gotthard Road Tunnel]] in [[Switzerland]] in 2001. One of the worst railway disasters ever, the [[Balvano train disaster]], was caused by a train stalling in the Armi tunnel in [[Italy]] in 1944, killing 426 passengers. Designers try to reduce these risks by installing emergency ventilation systems or isolated emergency escape tunnels parallel to the main passage.

=== Project planning and cost estimates ===
Government funds are often required for the creation of tunnels.<ref>{{cite web|url=http://www.cord.edu/faculty/bfoss/ADVC20.htm |title=Capital Projects Funds |publisher=Cord.edu |access-date=19 April 2013 |url-status=dead |archive-url=https://web.archive.org/web/20111217080357/http://www.cord.edu/faculty/bfoss/ADVC20.htm |archive-date=17 December 2011 }}</ref> When a tunnel is being planned or constructed, economics and politics play a large factor in the decision making process. Civil engineers usually use [[project management]] techniques for developing a major structure. Understanding the amount of time the project requires, and the amount of labor and materials needed is a crucial part of project planning. The project duration must be identified using a [[work breakdown structure]] and [[critical path method]]. Also, the land needed for excavation and construction staging, and the proper machinery must be selected. Large infrastructure projects require millions or even billions of dollars, involving long-term financing, usually through issuance of [[Bond (finance)|bonds]].

The [[Cost–benefit analysis|costs and benefits]] for an infrastructure such as a tunnel must be identified. Political disputes can occur, as in 2005 when the US House of Representatives approved a $100 million federal grant to build a tunnel under New York Harbor. However, the [[Port Authority of New York and New Jersey]] was not aware of this bill and had not asked for a grant for such a project.<ref>{{cite news| url=https://www.nytimes.com/2005/08/03/nyregion/03rail.html?_r=0 | work=The New York Times | first=Sewell | last=Chan | title=$100 Million for a Tunnel. What Tunnel? | date=3 August 2005}}</ref> Increased taxes to finance a large project may cause opposition.<ref>{{cite web |url=http://www.cfr.org/infrastructure/encouraging-us-infrastructure-investment/p27771 |title=Encouraging U.S. Infrastructure Investment – Council on Foreign Relations |publisher=Cfr.org |access-date=19 April 2013 |archive-date=23 May 2013 |archive-url=https://web.archive.org/web/20130523092826/http://www.cfr.org/infrastructure/encouraging-us-infrastructure-investment/p27771 |url-status=dead }}</ref>

== Construction ==
{{Main|Tunnel construction}}
{{anchor|Construction}}
{{anchor|Cut-and-cover}}
Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the groundwater conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and the shape of the tunnel and appropriate risk management.

There are three basic types of tunnel construction in common use. Cut-and-cover tunnels are constructed in a shallow trench and then covered over. Bored tunnels are constructed in situ, without removing the ground above. Finally, a tube can be sunk into a body of water, which is called an immersed tunnel.

=== Cut-and-cover ===
<!--links from Charles Pearson --><!--linked from [[Cut-and-cover]]-->
[[File:Paris Metro construction 03300288-3.jpg|thumb|Cut-and-cover construction at [[Saint-Michel (Paris Métro)|Saint-Michel]] on [[Paris Métro]] [[Paris Métro Line 4|Line 4]] (c. 1910)]]

'''Cut-and-cover''' is a simple method of construction for shallow tunnels where a [[trench]] is excavated and [[roof]]ed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel.{{sfn|Ellis|2015|p=118}}

There are two basic forms of cut-and-cover tunnelling:
* ''Bottom-up method'': A trench is excavated, with ground support as necessary, and the tunnel is constructed in it. The tunnel may be of in situ concrete, precast concrete, precast arches, or corrugated steel arches; in early days brickwork was used. The trench is then carefully back-filled and the surface is reinstated.
* ''Top-down method'': Side support walls and capping beams are constructed from ground level by such methods as [[slurry wall]]ing or contiguous bored piling. Only a shallow excavation is needed to construct the tunnel roof using precast beams or in situ concrete sitting on the walls. The surface is then reinstated except for access openings. This allows early reinstatement of roadways, services, and other surface features. Excavation then takes place under the permanent tunnel roof, and the base slab is constructed.

Shallow tunnels are often of the cut-and-cover type (if under water, of the immersed-tube type), while deep tunnels are excavated, often using a [[tunnelling shield]]. For intermediate levels, both methods are possible.

Large cut-and-cover boxes are often used for underground [[Rapid transit|metro]] stations, such as [[Canary Wharf tube station]] in London. This construction form generally has two levels, which allows economical arrangements for ticket hall, station platforms, passenger access and emergency egress, ventilation and smoke control, staff rooms, and equipment rooms. The interior of Canary Wharf station has been likened to an underground cathedral, owing to the sheer size of the excavation. This contrasts with many traditional stations on [[London Underground]], where bored tunnels were used for stations and passenger access. Nevertheless, the original parts of the London Underground network, the Metropolitan and District Railways, were constructed using cut-and-cover. These lines pre-dated electric traction and the proximity to the surface was useful to ventilate the inevitable smoke and steam.

A major disadvantage of cut-and-cover is the widespread disruption generated at the surface level during construction.<ref>{{Cite journal |last=Konyukhov |first=D. S. |date=12 April 2022 |title=Analysis of mechanized tunneling parameters to determine the overcutting characteristics |url=https://mst.misis.ru/jour/article/view/330 |journal=Gornye Nauki i Tekhnologii = Mining Science and Technology (Russia) |volume=7 |issue=1 |pages=49–56 |doi=10.17073/2500-0632-2022-1-49-56 |s2cid=248136002 |issn=2500-0632|doi-access=free }}</ref> This, and the availability of electric traction, brought about London Underground's switch to bored tunnels at a deeper level towards the end of the 19th century.

Prior to the replacement of manual excavation by the use of boring machines, [[Victorian era|Victorian]] tunnel excavators developed a specialized method called clay-kicking for digging tunnels in clay-based soils. The clay-kicker lies on a plank at a 45-degree angle away from the working face and rather than a [[mattock]] with his hands, inserts with his feet a tool with a cup-like rounded end, then turns the tool with his hands to extract a section of soil, which is then placed on the waste extract.
Clay-kicking is a specialized method developed in the [[United Kingdom]] of digging tunnels in strong clay-based soil structures. This method of cut and cover construction required relatively little disturbance of property during the renewal of the United Kingdom's then ancient [[sewerage]] systems. It was also used during the [[First World War]] by [[Royal Engineer tunnelling companies]] placing mines beneath [[German Empire|German]] lines, because it was almost silent and so not susceptible to listening methods of detection.<ref>{{cite web |date=2009 |title=Tunnelling |url=http://tunnellersmemorial.com/Tunnelling.htm |url-status=dead |archive-url=https://web.archive.org/web/20100823081917/http://tunnellersmemorial.com/Tunnelling.htm |archive-date=23 August 2010 |access-date=20 June 2010 |website=The Tunneller's Memorial, Givenchy |publisher=}}</ref>

=== Boring machines ===
{{Main|Tunnel boring machine}}
[[File:TBM S-210 Alptransit Faido East.jpg|thumb|A workman is dwarfed by the cutting end of a [[tunnel boring machine]] used to excavate the [[Gotthard Base Tunnel]] ([[Switzerland]]), the world's longest railway tunnel.]]

[[Tunnel boring machine]]s (TBMs) and associated back-up systems are used to highly automate the entire tunnelling process, reducing tunnelling costs. In certain predominantly urban applications, tunnel boring is viewed as a quick and cost-effective alternative to laying surface rails and roads. Expensive [[eminent domain|compulsory purchase]] of buildings and land, with potentially lengthy planning inquiries, is eliminated. Disadvantages of TBMs arise from their usually large size – the difficulty of transporting the large TBM to the site of tunnel construction, or (alternatively) the high cost of assembling the TBM on-site, often within the confines of the tunnel being constructed.

There are a variety of TBM designs that can operate in a variety of conditions, from hard rock to soft water-bearing ground. Some TBMs, the bentonite slurry and earth-pressure balance types, have pressurized compartments at the front end, allowing them to be used in difficult conditions below the [[water table]]. This pressurizes the ground ahead of the TBM cutter head to balance the water pressure. The operators work in normal air pressure behind the pressurized compartment, but may occasionally have to enter that compartment to renew or repair the cutters. This requires special precautions, such as local ground treatment or halting the TBM at a position free from water. Despite these difficulties, TBMs are now preferred over the older method of tunnelling in compressed air, with an airlock/decompression chamber some way back from the TBM, which required operators to [[work in compressed air|work in high pressure]] and go through decompression procedures at the end of their shifts, much like [[underwater diving|deep-sea divers]].

In February 2010, Aker Wirth delivered a TBM to Switzerland, for the expansion of the [[Linth–Limmern Power Stations]] located south of [[Linthal, Glarus|Linthal]] in the [[canton of Glarus]]. The borehole has a diameter of {{convert|8.03|m|ft}}.<ref>{{cite web |title=Tunnels & Tunnelling International |url=http://www.tunnelsonline.info/story.asp?sectioncode=1&storycode=61846 |url-status=dead |archive-url=https://web.archive.org/web/20120316162001/http://www.tunnelsonline.info/story.asp?sectioncode=1&storycode=61846 |archive-date=16 March 2012 |access-date=19 April 2013 |website=Tunnelsonline.info |publisher=}}</ref> The four TBMs used for excavating the {{convert|57|km|mi|adj=on}} [[Gotthard Base Tunnel]], in [[Switzerland]], had a diameter of about {{convert|8.8-9.6|m|ft}}. A larger TBM was built to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the [[HSL-Zuid]] in the Netherlands, with a diameter of {{convert|14.87|m|ft|1}}.<ref>{{cite web |title=The Groene Hart Tunnel |url=http://www.hslzuid.nl/hsl/uk/bouw/ment/Bored_Tunnel_Groene_Hart/index.jsp |url-status=dead |archive-url=https://web.archive.org/web/20090925160241/http://www.hslzuid.nl/hsl/uk/bouw/ment/Bored_Tunnel_Groene_Hart/index.jsp |archive-date=25 September 2009 |access-date=19 April 2013 |website=Hslzuid.nl |publisher=}}</ref> This in turn was superseded by the [[Autopista de Circunvalación M-30|Madrid M30 ringroad]], Spain, and the [[Shanghai Yangtze River Tunnel and Bridge|Chong Ming]] tunnels in [[Shanghai]], China. All of these machines were built at least partly by [[Herrenknecht]]. {{As of|2013|August|}}, the world's largest TBM was "[[Bertha (tunnel boring machine)|Big Bertha]]", a {{convert|57.5|ft|adj=on|order=flip}} diameter machine built by [[Hitachi Zosen Corporation]], which dug the [[Alaskan Way Viaduct replacement tunnel]] in [[Seattle, Washington]] (US).<ref name="NYT Dec 2012">{{cite news |last=Johnson |first=Kirk |date=5 December 2012 |title=Engineering Projects Will Transform Seattle, All Along the Waterfront |url=https://www.nytimes.com/2012/12/05/us/projects-to-transform-seattle-all-along-the-waterfront.html |url-access=subscription |access-date=2024-01-23 |newspaper=[[The New York Times]]}}</ref>

=== Shafts ===
[[File:Mersey Railway Tunnel - ventilation and drainage machinery.png|right|thumb|1886 illustration showing the ventilation and drainage system of the Mersey railway tunnel]]
A temporary access [[Shaft (civil engineering)|shaft]] is sometimes necessary during the excavation of a tunnel. They are usually circular and go straight down until they reach the level at which the tunnel is going to be built. A shaft normally has concrete walls and is usually built to be permanent. Once the access shafts are complete, TBMs are lowered to the bottom and excavation can start. Shafts are the main entrance in and out of the tunnel until the project is completed. If a tunnel is going to be long, multiple shafts at various locations may be bored so that entrance to the tunnel is closer to the unexcavated area.<ref name="Engineers. 1978">United States Army Corps of Engineers. (1978). Tunnels and shafts in rock. Washington, DC: Department of the Army.</ref>

Once construction is complete, construction access shafts are often used as [[ventilation shaft]]s, and may also be used as emergency exits.

=== Sprayed concrete techniques ===
The [[New Austrian tunneling method|new Austrian tunnelling method]] (NATM)—also referred to as the Sequential Excavation Method (SEM)<ref name=tbm20181205/>—was developed in the 1960s.
The main idea of this method is to use the geological [[stress (physics)|stress]] of the surrounding [[Rock (geology)|rock]] [[mass]] to stabilize the tunnel, by allowing a measured relaxation and stress reassignment into the surrounding rock to prevent full loads becoming imposed on the supports. Based on [[geotechnical]] measurements, an optimal [[cross section (geometry)|cross section]] is computed. The excavation is protected by a layer of sprayed concrete, commonly referred to as [[shotcrete]]. Other support measures can include steel arches, rock bolts, and mesh. Technological developments in sprayed concrete technology have resulted in steel and polypropylene fibers being added to the concrete mix to improve lining strength. This creates a natural load-bearing ring, which minimizes the rock's [[Deformation (mechanics)|deformation]].<ref name=tbm20181205/>

[[File:Hill 60 illowra battery port kembla.jpg|thumb|[[Illowra Battery]] utility tunnel, Port Kembla. One of many [[Template:Barracks Batteries Bunkers and Forts in Sydney|bunkers south of Sydney]].]]

By special [[Condition monitoring|monitoring]] the NATM method is flexible, even at surprising changes of the [[geomechanics|geomechanical]] rock consistency during the tunneling work. The measured rock properties lead to appropriate [[tool]]s for tunnel [[Strength of materials|strengthening]].<ref name="tbm20181205">
{{cite news |date=5 December 2018 |title=Understanding the New Austrian Tunnel Method (NATM) |url=https://tunnelingonline.com/understanding-the-new-austrian-tunnel-method-natm/ |access-date=27 December 2018 |work=Tunnel Business Magazine |publisher=Benjamin Media}}</ref>

=== Pipe jacking ===
{{Main|Pipe jacking}}

In '''[[pipe jacking]]''', [[hydraulic jack]]s are used to push specially made pipes through the ground behind a TBM or shield. This method is commonly used to create tunnels under existing structures, such as roads or railways. Tunnels constructed by pipe jacking are normally small diameter bores with a maximum size of around {{convert|3.2|m}}.

=== Box jacking ===
Box jacking is similar to pipe jacking, but instead of jacking tubes, a box-shaped tunnel is used. Jacked boxes can be a much larger span than a pipe jack, with the span of some box jacks in excess of {{convert|20|m}}. A cutting head is normally used at the front of the box being jacked, and spoil removal is normally by excavator from within the box. Recent developments of the Jacked Arch and Jacked deck have enabled longer and larger structures to be installed to close accuracy.

=== Underwater tunnels ===
[[File:Georgiaaquariumtunnel.jpg|thumb|[[Shark tunnel]] at the [[Georgia Aquarium]]]]
{{Main|Undersea tunnel}}

There are also several approaches to underwater tunnels, the two most common being bored tunnels or [[immersed tube]]s, examples are [[Bjørvika Tunnel]] and [[Marmaray]]. [[Submerged floating tunnel]]s are a novel approach under consideration; however, no such tunnels have been constructed to date.

=== Temporary way ===
During construction of a tunnel it is often convenient to install a temporary railway, particularly to remove [[spoil tip|excavated spoil]], often [[narrow gauge]] so that it can be [[double track]] to allow the operation of empty and loaded trains at the same time. The temporary way is replaced by the [[permanent way]] at completion, thus explaining the term "[[Perway]]".

=== Enlargement ===
[[File:Kolektory Praha, 15.jpg|thumb|A utility tunnel in [[Prague]]]]

The vehicles or traffic using a tunnel can outgrow it, requiring replacement or enlargement:
* The original single line [[Mount Gibraltar|Gib Tunnel]] near [[Mittagong]] was replaced with a double-track tunnel, with the original tunnel used for growing mushrooms.<ref name="Company Website">{{cite web |last=Sun |first=Li |title=Mushrooms and Tours {{!}} Li-Sun Exotic Mushrooms |url=http://www.li-sunexoticmushrooms.com.au/ |url-status=dead |archive-url=https://web.archive.org/web/20180316053305/http://www.li-sunexoticmushrooms.com.au/ |archive-date=16 March 2018 |access-date=2024-01-23 |website=www.li-sunexoticmushrooms.com.au }}</ref><ref name="Article on Closure">{{cite web |last1=Biscoe |first1=Emma |date=15 May 2014 |title=Mittagong mushroom business will close down |url=https://www.illawarramercury.com.au/story/2284594/mittagong-mushroom-business-will-close-down/ |url-access=subscription |website=Illawarra Mercury |language=en}}</ref>
* The 1832 double-track {{convert|1|mi|km|adj=on|order=flip}}-long tunnel from [[Edge Hill railway station|Edge Hill]] to [[Liverpool Lime Street railway station|Lime Street]] in [[Liverpool]] was near totally removed, apart from a {{convert|50|m|yd|adj=on}} section at Edge Hill and a section nearer to Lime Street, as four tracks were required. The tunnel was dug out into a very deep four-track cutting, with short tunnels in places along the cutting. Train services were not interrupted as the work progressed.<ref>{{cite web |date=1881 |title=National Railway Museum / Science & Society image # 10445941 |url=https://www.scienceandsociety.co.uk/results.asp?image=10445941 |url-status=live |archive-url=https://web.archive.org/web/20160304114308/http://i34.tinypic.com/23ixthy.jpg |archive-date=4 March 2016 |access-date=2024-01-23 |website=Science & Society Picture Library}}</ref><ref>{{cite web |date=1881 |title=National Railway Museum / Science & Society image # 10445944: "Building a railway bridge in Liverpool, 1881" |url=https://www.scienceandsociety.co.uk/results.asp?image=10445944 |url-status=live |archive-url=https://web.archive.org/web/20160304185533/http://i36.tinypic.com/16k1ahx.jpg |archive-date=4 March 2016 |access-date=2023-01-24 |website=Science & Society Picture Library}}</ref> There are other occurrences of tunnels being replaced by open cuts, for example, the [[Auburn Tunnel]].
* The [[Farnworth Tunnel]] in England was enlarged using a [[tunnel boring machine]] (TBM) in 2015.<ref>{{Cite web |last= |first= |date=2015-08-12 |title=UK's biggest TBM rebores Farnworth Tunnel |url=https://www.railwaygazette.com/uks-biggest-tbm-rebores-farnworth-tunnel/41248.article |access-date=2024-01-23 |website=Railway Gazette International |language=en}}</ref> The [[Rhyndaston Tunnel]] was enlarged using a borrowed TBM so as to be able to take [[ISO container]]s.
* Tunnels can also be enlarged by lowering the floor.<ref>{{cite web|url=http://www.tunnel-online.info/en/artikel/tunnel_2012-04_Report_on_Redeveloping_Railway_Tunnels_1433844.html|title=Report on Redeveloping Railway Tunnels|website=www.tunnel-online.info}}</ref>

=== Open building pit ===
An open building pit consists of a horizontal and a vertical boundary that keeps groundwater and soil out of the pit. There are several potential alternatives and combinations for (horizontal and vertical) building pit boundaries. The most important difference with cut-and-cover is that the open building pit is muted after tunnel construction; no roof is placed.

=== Other construction methods ===
[[File:Tunnel end Onkalo.jpg|thumb|Tunnel excavated with drilling and blasting method]]

* [[Drilling and blasting]]
* [[Hydraulic splitter]]
* Slurry-shield machine
* Wall-cover construction method.

== Variant tunnel types ==

=== Double-deck and multipurpose tunnels ===
[[File:San Francisco-Bay Bridge02.jpg|thumb|The upper-level traffic lanes through [[Yerba Buena Island]], part of the [[San Francisco–Oakland Bay Bridge]]]]
Some tunnels are double-deck, for example, the two major segments of the [[San Francisco–Oakland Bay Bridge]] (completed in 1936) are linked by a {{convert|540|foot|m|adj=mid|order=flip}} [[Yerba Buena Tunnel|double-deck tunnel]] section through [[Yerba Buena Island]], the largest-diameter bored tunnel in the world.<ref>{{cite web |date=4 November 2015 |title=San Francisco-Oakland Bay Bridge |url=http://mtc.ca.gov/about-mtc/what-mtc/bay-area-toll-authority/san-francisco-oakland-bay-bridge |archive-url=https://web.archive.org/web/20161107072916/http://mtc.ca.gov/about-mtc/what-mtc/bay-area-toll-authority/san-francisco-oakland-bay-bridge |archive-date=2016-11-07 |access-date=2024-01-23 |work=Bay Area Toll Authority}}</ref> At construction this was a combination bidirectional rail and truck pathway on the lower deck with automobiles above, now converted to one-way road vehicle traffic on each deck.

In Turkey, the [[Eurasia Tunnel]] under the [[Bosphorus]], opened in 2016, has at its core a {{convert|5.4|km|mile|abbr=on}} two-deck road tunnel with two lanes on each deck.<ref name="uc">{{cite web |title=Eurasia Tunnel Project |url=http://events.unicredit-cib.eu/uploads/media/Presentation_Basar_Arioglu_Yapi_Merkezi_Insaat.pdf |url-status=unfit |archive-url=https://web.archive.org/web/20160120170635/http://events.unicredit-cib.eu/uploads/media/Presentation_Basar_Arioglu_Yapi_Merkezi_Insaat.pdf |archive-date=20 January 2016 |access-date=13 April 2014 |publisher=[[Unicredit]] – Yapı Merkezi, SK EC Joint Venture}}</ref>

Additionally, in 2015 the Turkish government announced that it will build ''three''-level tunnel, also under the Bosporus.<ref>{{cite web |date=2015-02-27 |title=Istanbul's mega-project: World's first three-level tunnel to be built under the Bosporus – Daily Sabah |url=https://www.dailysabah.com/business/2015/02/27/istanbuls-megaproject-worlds-first-threelevel-tunnel-to-be-built-under-the-bosporus |url-status=live |archive-url=https://web.archive.org/web/20170219015521/https://www.dailysabah.com/business/2015/02/27/istanbuls-megaproject-worlds-first-threelevel-tunnel-to-be-built-under-the-bosporus |archive-date=19 February 2017 |access-date=2024-01-23 |website=[[Daily Sabah]]}}</ref> The tunnel is intended to carry both the Istanbul metro and a two-level highway, over a length of {{convert|6.5|km|mile|abbr=on}}.

The French {{ill|A86 Duplex Tunnel|fr|Duplex A86}} in west Paris consists of two bored tunnel tubes, the eastern one of which has two levels for light motorized vehicles, over a length of {{convert|10|km|mile|abbr=on}}. Although each level offers a physical height of {{convert|2.54|m|foot|abbr=on}}, only traffic up to {{convert|2|m|foot|abbr=on}} tall is allowed in this tunnel tube, and motorcyclists are directed to the other tube. Each level was built with a three-lane roadway, but only two lanes per level are used – the third serves as a hard shoulder within the tunnel. The A86 Duplex is Europe's longest double-deck tunnel.

In [[Shanghai]], China, a {{convert|2.8|km|mile|abbr=on}} two-tube double-deck tunnel was built starting in 2002. In each tube of the {{ill|Fuxing Road Tunnel|zh|复兴东路隧道}} both decks are for motor vehicles. In each direction, only cars and taxis travel on the {{convert|2.6|m|foot|abbr=on}} high two-lane upper deck, and heavier vehicles, like trucks and buses, as well as cars, may use the {{convert|4.0|m|foot|abbr=on}} high single-lane lower level.<ref>{{cite web |date=2002-11-01 |title=Shanghai to Dig Double-deck Tunnel |url=http://www.china.org.cn/english/China/47409.htm |url-status=live |archive-url=https://web.archive.org/web/20060515033120/http://www.china.org.cn/english/China/47409.htm |archive-date=15 May 2006 |access-date=2024-01-23 |website=China Facts and Figures 2002}}</ref>

In the Netherlands, a {{convert|2.3|km|mile|abbr=on}} two-storey, eight-lane, cut-and-cover road tunnel under the city of [[Maastricht]] was opened in 2016.<ref>{{cite web |last=Kalus |first=Ruben |date=2016-12-22 |title=Maastricht's new highway tunnel: a role model for Europe |url=http://www.dw.com/en/maastrichts-new-highway-tunnel-a-role-model-for-europe/av-36867098 |access-date=2024-01-23 |website=DW.COM}}</ref> Each level accommodates a full height, two by two-lane highway. The two lower tubes of the tunnel carry the [[A2 motorway (Netherlands)|A2 motorway]], which originates in Amsterdam, through the city; and the two upper tubes take the N2 regional highway for local traffic.<ref>{{Cite web |date=2015-08-17 |title=Unique stacked tunnel under Maastricht - Division Traffic & Infra |url=http://imtech.com/EN/traffic-infra/Traffic-Infra-Newsroom/Divisie-Imtech-Traffic-Infra-Homepage-Newsroom-Highlights/Unique-stacked-tunnel-under-Maastricht.html |url-status=dead |archive-url=https://web.archive.org/web/20150817110627/http://imtech.com/EN/traffic-infra/Traffic-Infra-Newsroom/Divisie-Imtech-Traffic-Infra-Homepage-Newsroom-Highlights/Unique-stacked-tunnel-under-Maastricht.html |archive-date=2015-08-17 |access-date=2024-01-23 |website=Imtech}}</ref>

The [[Alaskan Way Viaduct replacement tunnel]], is a $3.3&nbsp;billion {{convert|1.76|mi|adj=on|order=flip}}, double-decker bored highway tunnel under [[Downtown Seattle]]. Construction began in July 2013 using "[[Bertha (tunnel boring machine)|Bertha]]", at the time the world's largest [[earth pressure balance]] tunnel boring machine, with a {{convert|57.5|ft|adj=on|order=flip}} cutterhead diameter. After several delays, tunnel boring was completed in April 2017, and the tunnel opened to traffic on 4 February 2019.

[[New York City]]'s [[63rd Street Tunnel]] under the [[East River]], between the boroughs of [[Manhattan]] and [[Queens]], was intended to carry [[New York City Subway|subway]] trains on the upper level and [[Long Island Rail Road]] commuter trains on the lower level. Construction started in 1969,<ref>{{cite news|title=To Break Ground For 63rd St., East River Tunnel|url=http://fultonhistory.com/highlighter/highlight-for-xml?altUrl=http%3A%2F%2Ffultonhistory.com%2FNewspapers%252023%2FForest%2520Parkway%2520NY%2520Leader%2520Observer%2FForest%2520Parkway%2520NY%2520Leader%2520Observer%25201969-1971%2FForest%2520Parkway%2520NY%2520Leader%2520Observer%25201969-1971%2520-%25200628.pdf&highlightsFile=http%3A%2F%2Ffultonhistory.com%2Fhighlighter%2Fhits%2F816c7feba198b14dee5deb0dc12d134f#page=1|access-date=29 July 2016|work=New York Leader-Observer|via=[[Old Fulton New York Postcards]]|date=20 November 1969|page=8}}</ref> and the two sides of the tunnel were bored through in 1972.<ref>{{Cite news |date=11 October 1972 |title=Governor Rockefeller and Mayor Lindsay Attend 'Holing Through' of 63d St. Tunnel |url=https://www.nytimes.com/1972/10/11/archives/governor-rockefeller-and-mayor-lindsay-attend-holing-through-of-63d.html |url-access=subscription |access-date=3 February 2018 |work=The New York Times |pages=47 |language=en-US |issn=0362-4331}}</ref> The upper level, used by the [[63rd Street lines|IND 63rd Street Line]] ({{NYCS trains|63rd IND}}) of the New York City Subway, was not opened for passenger service until 1989.<ref>{{cite news |last=Lorch |first=Donatella |date=29 October 1989 |title=The 'Subway to Nowhere' Now Goes Somewhere |url=https://www.nytimes.com/1989/10/29/nyregion/the-subway-to-nowhere-now-goes-somewhere.html |url-access=subscription |access-date=20 October 2011 |newspaper=The New York Times |pages=1.37}}</ref> The lower level, intended for commuter rail, saw passenger service after completion of the [[East Side Access]] project, in late 2022.<ref>{{Cite web |date=2023-10-05 |title=East Side Access |url=https://new.mta.info/project/east-side-access |access-date=2024-01-23 |website=MTA |language=en}}</ref>

In the UK, the 1934 [[Queensway Tunnel]] under the [[River Mersey]] between [[Liverpool]] and [[Birkenhead]] was originally to have road vehicles running on the upper deck and trams on the lower. During construction the tram usage was cancelled. The lower section is only used for cables, pipes and emergency accident refuge enclosures.

Hong Kong's [[Lion Rock Tunnel]], built in the mid 1960s, connecting [[New Kowloon]] and [[Sha Tin New Town|Sha Tin]], carries a motorway but also serves as an [[Aqueduct (water supply)|aqueduct]], featuring a gallery containing five water mains lines with diameters between {{cvt|1.2|and|1.5|m|ft|0}} below the road section of the tunnel.<ref>{{cite web|url=https://www.waterworld.com/technologies/pipes/article/16216777/black-veatch-uses-trenchless-technology-for-water-main-rehabilitation-in-hong-kong|title=Black & Veatch uses trenchless technology for water main rehabilitation in Hong Kong – WaterWorld|date=5 June 2017|url-status=live|archive-url=https://web.archive.org/web/20170605114200/http://www.waterworld.com/articles/2009/08/black--veatch-uses.html|archive-date=5 June 2017}}</ref>

[[Wuhan]]'s Yangtze River Highway and Railway Tunnel is a {{cvt|2.59|km|mi}} two-tube double-deck tunnel under the [[Yangtze|Yangtze River]] completed in 2018. Each tube carries three lanes of local traffic on the top deck with one track [[Wuhan Metro]] [[Line 7 (Wuhan Metro)|Line 7]] on the lower deck.<ref>{{Cite web|title=新闻分析：这条隧道藏了多少科技秘密——揭秘"万里长江公铁第一隧"-新华网|url=http://www.xinhuanet.com/2018-10/01/c_129964700.htm|archive-url=https://web.archive.org/web/20190716063606/http://www.xinhuanet.com/2018-10/01/c_129964700.htm|url-status=dead|archive-date=16 July 2019|access-date=7 June 2020|website=www.xinhuanet.com}}</ref><ref>{{Cite web|last=汉网|date=27 September 2018|title=揭秘武汉长江公铁第一隧"尖板眼" 68个通道可供江底逃生|url=https://news.sina.com.cn/o/2018-09-27/doc-ifxeuwwr8734437.shtml|access-date=7 June 2020|website=news.sina.com.cn}}</ref><ref>{{Cite web|title=武汉长江公铁隧道工程打造全球"超级工程"样板-新华网|url=http://m.xinhuanet.com/hb/2018-09/19/c_1123450459.htm|access-date=7 June 2020|website=m.xinhuanet.com}}</ref>

[[Mount Baker Tunnel]] has three levels. The bottom level is to be used by [[Sound Transit]] light rail. The middle level is used by car traffic, and the top layer is for bicycle and pedestrian access.
 
Some tunnels have more than one purpose. The [[SMART Tunnel]] in [[Malaysia]] is the first multipurpose "''Stormwater Management And Road Tunnel''" in the world, created to convey both traffic and occasional [[flood]] waters in [[Kuala Lumpur]]. When necessary, floodwater is first diverted into a separate bypass tunnel located ''underneath'' the {{convert|2.5|mile|km|abbr=on|order=flip}} double-deck roadway tunnel. In this scenario, traffic continues normally. Only during heavy, prolonged rains when the threat of extreme flooding is high, the upper tunnel tube is closed off to vehicles and automated flood control gates are opened so that water can be diverted through both tunnels.<ref>{{cite web|url=https://www.mnn.com/green-tech/transportation/stories/drive-through-these-10-tremendous-tunnels|title=Drive through these 10 tremendous tunnels}}</ref>

=== Covered passageways ===
[[File:Sotetkapu.JPG|right|thumb|upright|The 19th-century [[Dark Gate]] in [[Esztergom]], Hungary]]

Over-bridges can sometimes be built by covering a road or river or railway with [[brick]] or [[steel]] [[arch]]es, and then levelling the surface with earth. In railway parlance, a surface-level track which has been built or covered over is normally called a "covered way".

[[Snow shed]]s are a kind of artificial tunnel built to protect a railway from [[avalanche]]s of snow. Similarly the [[Stanwell Park]], [[New South Wales]] "steel tunnel", on the [[Illawarra railway line]], protects the line from rockfalls.

=== Underpass ===
{{main|Subway (crossing)}}
[[File:VIEW THROUGH CATTLEPASS NEAR BRIDAL VEIL FALLS. - Historic Columbia River Highway, Troutdale, Multnomah County, OR HAER ORE,26-TROUT.V,1-31.tif|right|thumb|Underpass for cattle created in 1914 construction of what is now [[Historic Columbia River Highway]]]]
An '''underpass''' is a road or railway or other passageway passing ''under'' another road or railway, under an [[overpass]]. This is not strictly a tunnel.

=== Utility Tunnel ===
A [[Utility tunnel|Utility Tunnel]] is built for the purpose of carrying one or more [[Public utility|utilities]] in the same space, for this reason they are also referred to as Multi-Utility Tunnels or MUTs. Through co-location of different utilities in one tunnel, organizations are able to reduce the financial and environmental costs of building and maintaining utilities.<ref>{{Cite journal |last1=Hunt |first1=D. V. L. |last2=Nash |first2=D. |last3=Rogers |first3=C. D. F. |date=2014-01-01 |title=Sustainable utility placement via Multi-Utility Tunnels |url=https://www.sciencedirect.com/science/article/pii/S0886779812000338 |journal=Tunnelling and Underground Space Technology |series=Underground Infrastructure Research 2012 |volume=39 |pages=15–26 |doi=10.1016/j.tust.2012.02.001 |bibcode=2014TUSTI..39...15H |issn=0886-7798|doi-access=free }}</ref> These tunnels can be used for many types of utilities, routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment.<ref name=":0" />

== Safety and security ==
{{Redirect|Tunnel fire|the 1991 wildfire in California|Oakland firestorm of 1991|the 2022 wildfire in Arizona|Tunnel Fire (2022)}}
[[File:Alma tunnel Paris.jpg|thumb|left|The entrance to the [[Pont de l'Alma]] tunnel, the site where the car carrying [[Diana, Princess of Wales]], [[Death of Diana, Princess of Wales|hit a Fiat and then the wall]]. There was no proper barrier and this contributed to her death.]]

Owing to the enclosed space of a tunnel, fires can have very serious effects on users. The main dangers are gas and smoke production, with even low concentrations of [[carbon monoxide]] being highly toxic. Fires killed 11 people in the [[Gotthard Road Tunnel#History|Gotthard tunnel fire]] of 2001 for example, all of the victims succumbing to smoke and gas inhalation. Over 400 passengers died in the [[Balvano train disaster]] in Italy in 1944, when the locomotive halted in a long tunnel. [[Carbon monoxide poisoning]] was the main cause of death. In the [[Caldecott Tunnel fire]] of 1982, the majority of fatalities were caused by toxic smoke, rather than by the initial crash. Likewise 84 people were killed in the [[Paris Métro train fire]] of 1904.

Motor vehicle tunnels usually require [[ventilation shaft]]s and powered fans to remove toxic [[exhaust gas]]es during routine operation.<ref name="Mishra">{{cite journal
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Rail tunnels usually require fewer [[air changes per hour]], but still may require [[Ventilation (architecture)|forced-air ventilation]]. Both types of tunnels often have provisions to increase ventilation under emergency conditions, such as a fire. Although there is a risk of increasing the [[rate of combustion]] through increased airflow, the primary focus is on providing breathable air to persons trapped in the tunnel, as well as [[firefighter]]s.

The [[Aerodynamics|aerodynamic]] [[Longitudinal wave|pressure wave]] produced by [[High-speed rail|high speed trains]] entering a tunnel<ref>{{Cite journal|last1=Kim|first1=Joon-Hyung|last2=Rho|first2=Joo-Hyun|date=1 March 2018|title=Pressure wave characteristics of a high-speed train in a tunnel according to the operating conditions|journal=Proceedings of the Institution of Mechanical Engineers, Part F: Journal of Rail and Rapid Transit|language=en|volume=232|issue=3|pages=928–935|doi=10.1177/0954409717702015|s2cid=125620030|issn=0954-4097}}</ref> reflect at its open ends and change sign ([[Compression (physics)|compression]] wavefront changes to [[rarefaction]] wavefront and vice versa). When two wavefronts of the same sign meet the train, significant and rapid air pressure<ref>{{Cite journal|last1=Niu|first1=Jiqiang|last2=Zhou|first2=Dan|last3=Liu|first3=Feng|last4=Yuan|first4=Yanping|date=1 October 2018|title=Effect of train length on fluctuating aerodynamic pressure wave in tunnels and method for determining the amplitude of pressure wave on trains|journal=Tunnelling and Underground Space Technology|volume=80|pages=277–289|doi=10.1016/j.tust.2018.07.031|bibcode=2018TUSTI..80..277N | s2cid=116606435 |issn=0886-7798}}</ref> may cause ear discomfort<ref>{{Cite journal|last1=Xie|first1=Pengpeng|last2=Peng|first2=Yong|last3=Wang|first3=Tiantian|last4=Zhang|first4=Honghao|date=April 2019|title=Risks of Ear Complaints of Passengers and Drivers While Trains Are Passing Through Tunnels at High Speed: A Numerical Simulation and Experimental Study|journal=International Journal of Environmental Research and Public Health|volume=16|issue=7|pages=1283|doi=10.3390/ijerph16071283|issn=1661-7827|pmc=6480231|pmid=30974822|doi-access=free}}</ref> for passengers and crew. When a high-speed train exits a tunnel, a loud "[[Piston effect#Tunnel boom|Tunnel boom]]" may occur, which can disturb residents near the mouth of the tunnel, and it is exacerbated in mountain valleys where the sound can echo.

When there is a parallel, separate tunnel available, airtight but unlocked emergency doors are usually provided which allow trapped personnel to escape from a smoke-filled tunnel to the parallel tube.<ref>{{cite journal | last1 = Fridolf | first1 = K. | last2 = Ronchi | first2 = E. | last3 = Nilsson | first3 = D. | last4 = Frantzich | first4 = H. | year = 2013 | title = Movement speed and exit choice in smoke-filled rail tunnels | journal = Fire Safety Journal | volume = 59 | pages = 8–21 | doi = 10.1016/j.firesaf.2013.03.007 | bibcode = 2013FirSJ..59....8F }}</ref>

Larger, heavily used tunnels, such as the [[Big Dig]] tunnel in [[Boston, Massachusetts]], may have a dedicated 24-hour staffed [[operations center]] which monitors and reports on traffic conditions, and responds to emergencies.<ref>{{cite journal|last=Johnson|first=Christine M.|author2=Edward L. Thomas|title=A Case Study Boston Central Artery/Tunnel Integrated Project Control System, Responding to incidents Rapidly and Effectively|journal=Metropolitan Transportation Management Center|date=October 1999|page=12|url=http://ntl.bts.gov/lib/jpodocs/repts_te/11063.pdf|access-date=4 April 2014|archive-date=9 March 2013|archive-url=https://web.archive.org/web/20130309164537/http://ntl.bts.gov/lib/jpodocs/repts_te/11063.pdf|url-status=dead}}</ref> [[Video surveillance]] equipment is often used, and real-time pictures of traffic conditions for some highways may be viewable by the general public via the Internet.

A database of seismic damage to underground structures using 217 case histories shows the following general observations can be made regarding the seismic performance of underground structures:

* Underground structures suffer appreciably less damage than surface structures.
* Reported damage decreases with increasing over burden depth. Deep tunnels seem to be safer and less vulnerable to earthquake shaking than are shallow tunnels.
* Underground facilities constructed in soils can be expected to suffer more damage compared to openings constructed in competent rock.
* Lined and grouted tunnels are safer than unlined tunnels in rock. Shaking damage can be reduced by stabilizing the ground around the tunnel and by improving the contact between the lining and the surrounding ground through grouting.
* Tunnels are more stable under a symmetric load, which improves ground-lining interaction. Improving the tunnel lining by placing thicker and stiffer sections without stabilizing surrounding poor ground may result in excess seismic forces in the lining. Backfilling with non-cyclically mobile material{{clarify|What is non-cyclically mobile material?|date=October 2021}} and rock-stabilizing measures may improve the safety and stability of shallow tunnels.
* Damage may be related to peak ground acceleration and velocity based on the magnitude and epicentral distance of the affected earthquake.
* Duration of strong-motion shaking during earthquakes is of utmost importance because it may cause fatigue failure and therefore, large deformations.
* High frequency motions may explain the local spalling of rock or concrete along planes of weakness. These frequencies, which rapidly attenuate with distance, may be expected mainly at small distances from the causative fault.
* [[Ground motion]] may be amplified upon incidence with a tunnel if wavelengths are between one and four times the tunnel diameter.
* Damage at and near tunnel portals may be significant due to slope instability.<ref>{{Cite journal |last1=Hashash |first1=Youssef M.A. |last2=Hook |first2=Jeffrey J. |last3=Schmidt |first3=Birger |last4=Yao |first4=John I-Chiang |date=2001 |title=Seismic design and analysis of underground structures |url=https://www.sciencedirect.com/science/article/abs/pii/S0886779801000517 |journal=Tunnelling and Underground Space Technology |volume=16 |issue=4 |pages=247–293 |bibcode=2001TUSTI..16..247H |doi=10.1016/S0886-7798(01)00051-7 |s2cid=108456041 |via=[[Science Direct]]|url-access=subscription }}</ref>

Earthquakes are one of nature's most formidable threats. A magnitude 6.7 earthquake shook the San Fernando valley in Los Angeles in 1994. The earthquake caused extensive damage to various structures, including buildings, freeway overpasses and road systems throughout the area. The National Center for Environmental Information estimates total damages to be 40 billion dollars.<ref>{{cite web|url=https://www.ngdc.noaa.gov/hazel/view/hazards/earthquake/event-more-info/5372|title=Significant Earthquake Information|last=National Geophysical Data Center / World Data Service (NGDC/WDS): NCEI/WDS Global Significant Earthquake Database. NOAA National Centers for Environmental Information|year=1972 |publisher=NOAA National Centers for Environmental Information|doi=10.7289/V5TD9V7K}}</ref> According to an article issued by Steve Hymon of TheSource – Transportation News and Views, there was no serious damage sustained by the LA subway system. Metro, the owner of the LA subway system, issued a statement through their engineering staff about the design and consideration that goes into a tunnel system. Engineers and architects perform extensive analysis as to how hard they expect earthquakes to hit that area. All of this goes into the overall design and flexibility of the tunnel.

This same trend of limited subway damage following an earthquake can be seen in many other places. In 1985 a magnitude 8.1 earthquake shook Mexico City; there was no damage to the subway system, and in fact the subway systems served as a lifeline for emergency personnel and evacuations. A magnitude 7.2 ripped through Kobe Japan in 1995, leaving no damage to the tunnels themselves. Entry portals sustained minor damages, however these damages were attributed to inadequate earthquake design that originated from the original construction date of 1965. In 2010 a magnitude 8.8, massive by any scale, afflicted Chile. Entrance stations to subway systems suffered minor damages, and the subway system was down for the rest of the day. By the next afternoon, the subway system was operational again.<ref>Hymon, Steve. "Designing A Subway to Withstand an Earthquake." The Source. N.p., 2017. Web. 11 November 2017. http://thesource.metro.net/2012/08/10/designing-a-subway-to-withstand-an-earthquake/ {{Webarchive|url=https://web.archive.org/web/20240221224328/https://thesource.metro.net/2012/08/10/designing-a-subway-to-withstand-an-earthquake/ |date=21 February 2024 }}</ref>

== Examples==
{{Cleanup|section|reason=just a random list|date=December 2017}}

=== In history ===
{{See also|History of rapid transit}}
{{More citations needed section|date=March 2013}}
[[File:Dudley Canal Tunnel Southern Portal.jpg|thumb|right|Southern portal of the 1791 Dudley Canal tunnel in England]]
[[File:Entrance of the Railway at Edge Hill, from Bury's Liverpool and Manchester Railway, 1831 - artfinder 122456.jpg|right|thumb|[[Thomas Talbot Bury]]'s watercolour of the Liverpool Edge Hill tunnel portals]]
[[File:Start of Lime Street Tunnel Cutting.jpg|right|thumb|A short section remains of the 1832 Edge Hill to Lime Street tunnel in [[Liverpool]]. This and a short section of the original tunnel nearer to Lime Street are the oldest rail tunnels in the world still in active use.]]
[[File:Leaving Liverpool Lime St - geograph.org.uk - 747034.jpg|thumb|right|[[Liverpool Lime Street railway station|Liverpool Lime Street]] Approach. The original two-track tunnel was removed to create a deep cutting. Some of the road bridges seen across the cutting are solid rock and in effect are a series of short tunnels.]]
[[File:Edge Hill cutting.jpg|thumb|right|The three eastern portals of Liverpool Edge Hill tunnels, built into a hand-dug deep cutting. The left tunnel with tracks is the short 1846 second Crown Street Tunnel, still used for shunting. In the center, partially hidden by undergrowth, is the disused {{convert|2.03|km|mi|abbr=on}} 1829 [[Wapping Tunnel]]. On the right, hidden by undergrowth, is the disused original short 1829 Crown Street Tunnel.]]
[[File:Donner Pass Summit Tunnel West Portal.jpg|right|thumb|The 1,659-foot (506 m) [[Donner Pass]] Summit Tunnel (#6) was in service from 1868 to 1993.]]
<!-- [[File:Lehigh Tunnel.jpg|thumb|[[Lehigh Tunnel]], [[Pennsylvania]]]] not particularly notable?? -->
[[File:Sommeiller drill.jpg|thumb|A late-19th-century pneumatic rock-drilling machine, invented by [[Germain Sommeiller]] and used to drill the first large tunnels through the [[Alps]]]]
[[File:Small operational brick tunnel.JPG|thumb|Small operational brick tunnel in France]]

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The history of ancient tunnels and tunneling in the world is reviewed in various sources which include many examples of these structures that were built for different purposes.<ref>Klaus Grewe, 1998, Licht am Ende des Tunnels – Planung und Trassierung im antiken Tunnelbau, Verlag Philipp von Zabern, Mainz am Rhein.</ref><ref>Siamak Hashemi, 2013, The Magnificence of Civilization in Depths of Ground (A Review of Underground Structures in Iran – Past to Present), Shadrang Printing and Publishing Co., Tehran.</ref> Some well known ancient and modern tunnels are briefly introduced below:

* The [[qanat]] or [[kareez]] of [[Persia]] are water management systems used to provide a reliable supply of water to human settlements or for irrigation in hot, arid and semi-arid climates. The deepest known qanat is in the Iranian city of [[Gonabad]], which after 2700 years, still provides drinking and agricultural water to nearly 40,000 people. Its main well depth is more than {{convert|360|m|ft|abbr=on}}, and its length is {{convert|45|km|mi|abbr=on}}.<ref>UNESCO World Heritage Centre – World Heritage List: Qanats of Gonabad, Date of Inscription 2007, Reference No. 5207, At: {{cite web|url=https://whc.unesco.org/fr/listesindicatives/5207 |title=Qanats of Gonabad – UNESCO World Heritage Centre |access-date=14 December 2013 |url-status=live |archive-url=https://web.archive.org/web/20160329120845/https://whc.unesco.org/fr/listesindicatives/5207 |archive-date=29 March 2016 }}</ref>
* The [[Siloam Tunnel]] was built before 701&nbsp;BC for a reliable supply of water, to withstand [[siege]] attacks.
* The [[Eupalinian aqueduct]] on the island of [[Samos Island|Samos]] ([[North Aegean]], Greece) was built in 520&nbsp;BC by the ancient Greek engineer [[Eupalinos]] of [[Megara]] under a contract with the local community. Eupalinos organised the work so that the tunnel was begun from both sides of Mount Kastro. The two teams advanced simultaneously and met in the middle with excellent accuracy, something that was extremely difficult in that time. The aqueduct was of utmost defensive importance, since it ran underground, and it was not easily found by an enemy who could otherwise cut off the water supply to [[Pythagoreion]], the ancient capital of [[Samos Island|Samos]]. The tunnel's existence was recorded by Herodotus (as was the mole and harbour, and the third wonder of the island, the great temple to Hera, thought by many to be the largest in the Greek world). The precise location of the tunnel was only re-established in the 19th century by German archaeologists. The tunnel proper is {{convert|1030|m|ft|long|abbr=on|adj=mid}} and visitors can still enter it.
* One of the first known drainage and sewage networks in form of [[Drainage tunnel|tunnels]] was constructed at Persepolis in Iran at the same time as the construction of its foundation in 518&nbsp;BC. In most places the network was dug in the sound rock of the mountain and then covered by large pieces of rock and stone followed by earth and piles of rubble to level the ground. During investigations and surveys, long sections of similar rock tunnels extending beneath the palace area were traced by Herzfeld and later by Schmidt and their archaeological teams.<ref>Schmidt, E.F., 1953, Persepolis I – Structures, Reliefs, Inscriptions; The University of Chicago Oriental Institute Publications, Volume LXVIII, The University of Chicago Press.</ref>
* The [[Via Flaminia]], an important [[Roman road]], penetrated the [[Furlo]] pass in the [[Apennines]] through a tunnel which emperor [[Vespasian]] had ordered built in 76–77&nbsp;AD. A modern road, the SS 3 Flaminia, still uses this tunnel, which had a precursor dating back to the 3rd century BC, remnants of this earlier tunnel (one of the first road tunnels) are also still visible.
* The world's oldest tunnel traversing under a water body is claimed<ref>Blogcu {{cite web|url=http://terelek.blogcu.com/terelek-kaya-tuneli/319497 |title=TERELEK KAYA TÜNELİ – terelek |work=Blogcu |access-date=17 July 2014 |url-status=live |archive-url=https://web.archive.org/web/20160327222549/http://terelek.blogcu.com/terelek-kaya-tuneli/319497 |archive-date=27 March 2016 }}</ref> to be the ''Terelek kaya tüneli'' under [[Kızıl River]], a little south of the towns of [[Boyabat]] and [[Durağan]] in [[Turkey]], just downstream from where [[Kizil River]] joins its tributary [[Gökırmak]]. The tunnel is presently under a narrow part of a lake formed by a dam some kilometres further downstream. Estimated to have been built more than 2,000 years ago, possibly by the same civilization that also built the royal tombs in a rock face nearby, it is assumed to have had a defensive purpose.
* [[Sapperton Canal Tunnel]] on the [[Thames and Severn Canal]] in [[England]], dug through hills, which opened in 1789, was {{convert|3.5|km|mi|abbr=on}} long and allowed [[Barge|boat]] transport of [[coal]] and other goods. Above it the [[Sapperton railway tunnel|Sapperton Long Tunnel]] was constructed which carries the "Golden Valley" railway line between [[Swindon]] and [[Gloucester]].
* The 1791 [[Dudley Tunnel|Dudley canal tunnel]] is on the [[Dudley Canal]], in [[Dudley]], [[England]]. The tunnel is {{convert|1.83|mi|km|1|order=flip}} long. Closed in 1962 the tunnel was reopened in 1973. The series of tunnels was extended in 1984 and 1989.<ref>[http://www.dudleycanaltrust.org.uk/interactive-tour-map/ Map of Dudley Canals | Discover Black Country Canals] {{webarchive |url=https://web.archive.org/web/20150409192053/http://www.dudleycanaltrust.org.uk/interactive-tour-map/ |date=9 April 2015 }}</ref>
* [[Fritchley Tunnel]], constructed in 1793 in Derbyshire by the [[Butterley Company]] to transport limestone to its ironworks factory. The Butterley company engineered and built its own railway. A victim of the depression the company closed after 219 years in 2009. The tunnel is the world's oldest railway tunnel traversed by rail wagons. Gravity and horse haulage was utilised. The railway was converted to steam locomotion in 1813 using a [[Steam Horse locomotive]] engineered and built by the Butterley company, however reverted to horses. Steam trains used the tunnel continuously from the 1840s when the railway was converted to a narrow gauge. The line closed in 1933. In the Second World War, the tunnel was used as an air raid shelter. Sealed up in 1977 it was rediscovered in 2013 and inspected. The tunnel was resealed to preserved the construction as it was designated an ancient monument.<ref name=listedbuilding1422984>{{NHLE|num=1422984|desc=Fritchley Tunnel, Butterley Gangroad|access-date=19 March 2015}}</ref><ref name=BBC22342239>{{cite news|url=https://www.bbc.co.uk/news/uk-england-derbyshire-22342239|title=Archaeologists find 'world's oldest railway tunnel' in Derbyshire|date=1 May 2013|work=BBC News}}</ref>
* The 1794 [[Butterley Tunnel|Butterley canal tunnel]] canal tunnel is {{cvt|2819|m|mi|1}} in length on the [[Cromford Canal]] in [[Ripley, Derbyshire]], England. The tunnel was built simultaneously with the 1793 Fritchley railway tunnel. The tunnel partially collapsed in 1900 splitting the Cromford Canal, and has not been used since. The Friends of Cromford Canal, a group of volunteers, are working at fully restoring the Cromford Canal and the Butterley Tunnel.<ref>[http://www.cromfordcanal.info Friends of the Cromford Canal – HOME]{{webarchive|url=https://web.archive.org/web/20161023115647/http://www.cromfordcanal.info/ |date=23 October 2016 }}</ref>
* The 1796 Stoddart Tunnel in Chapel-en-le-Frith in [[Derbyshire]] is reputed to be the oldest rail tunnel in the world. The rail wagons were originally horse-drawn.
* [[Smuggling tunnel#Derby Tunnels|Derby Tunnels]] in [[Salem, Massachusetts]], were built in 1801 to smuggle imports affected by President [[Thomas Jefferson]]'s new customs duties. Jefferson had ordered local militias to help the Custom House in each port collect these dues, but the smugglers, led by Elias Derby, hired the Salem militia to dig the tunnels and hide the spoil.
* A tunnel was created for the first true steam locomotive, from [[Penydarren]] to [[Abercynon]]. The Penydarren locomotive was built by [[Richard Trevithick]]. The locomotive made the historic journey from Penydarren to Abercynon in 1804. Part of this tunnel can still be seen at [[Pentrebach]], [[Merthyr Tydfil]], [[Wales]]. This is arguably the oldest railway tunnel in the world, dedicated only to self-propelled steam engines on rails.
* The [[Montgomery Bell Tunnel]] in Tennessee, an {{convert|88|m|ft|abbr=on|adj=mid|-long}} water diversion tunnel, {{convert|4.50|x|2.45|m|ft|adj=mid|high|abbr=on}}, to power a water wheel, was built by slave labour in 1819, being the first full-scale tunnel in North America.
* Bourne's Tunnel, [[Rainhill]], near [[Liverpool]], England. It is {{convert|0.0321|km|m ft|abbr=on|order=out}} long. Built in the late 1820s, the exact date is unknown, however probably built in 1828 or 1829. This is the first tunnel in the world constructed under a railway line. The construction of the Liverpool to Manchester Railway ran over a horse-drawn tramway that ran from the Sutton collieries to the Liverpool-Warrington turnpike road. A tunnel was bored under the railway for the tramway. As the railway was being constructed the tunnel was made operational, opening prior to the Liverpool tunnels on the Liverpool to Manchester line. The tunnel was made redundant in 1844 when the tramway was dismantled.<ref>{{cite web|url=http://www.britishlistedbuildings.co.uk/en-507533-bourne-s-tunnel-at-sj5033491804-|title=Bourne's Tunnel at Sj5033491804 – Saint Helens – St Helens – England|work=British Listed Buildings|access-date=30 September 2014}}</ref>
* [[Crown Street railway station|Crown Street station]], [[Liverpool]], England, 1829. Built by [[George Stephenson]], a single-track railway tunnel, {{convert|266|m|ft|abbr=on}} long, was bored from Edge Hill to Crown Street to serve the world's first intercity passenger railway terminus station. The station was abandoned in 1836 being too far from Liverpool city centre, with the area converted for freight use. Closed down in 1972, the tunnel is disused. However it is the oldest passenger rail tunnel running under streets in the world.<ref>{{cite web |url=http://www.liverpoolwiki.org/Liverpool%27s_Historic_Rail_Tunnels |archive-url=https://web.archive.org/web/20090517123220/http://www.liverpoolwiki.org/Liverpool%27s_Historic_Rail_Tunnels |url-status=dead |archive-date=17 May 2009 |title=Liverpool's Historic Rail Tunnels |publisher=The Liverpool Wiki |date=22 February 1999 |access-date=19 April 2013 }}</ref><ref name="subbrit.org.uk">{{cite web|url=http://www.subbrit.org.uk/sb-sites/sites/l/liverpool_edge_hill_cutting/index3.shtml|title=Subterranea Britannica: Sites|access-date=30 September 2014}}</ref>
* The 1829 [[Wapping Tunnel]] in Liverpool, England, at {{convert|2.03|km|mi|abbr=on}} long on a twin track railway, was the first rail tunnel bored under a metropolis. The tunnel's path is from Edge Hill in the east of the city to [[Wapping Dock]] in the south end Liverpool docks. The tunnel was used only for freight terminating at the [[Park Lane railway goods station|Park Lane goods terminal]]. Currently disused since 1972, the tunnel was to be a part of the [[Merseyrail]] metro network, with work started and abandoned because of costs. The tunnel is in excellent condition and is still being considered for reuse by Merseyrail, maybe with an underground station cut into the tunnel for Liverpool university. The river portal is opposite the new [[Echo Arena Liverpool|King's Dock Liverpool Arena]] being an ideal location for a serving station. If reused the tunnel will be the oldest used underground rail tunnel in the world and oldest section of any underground metro system.<ref name="subbrit.org.uk"/><ref>{{cite web|url=http://www.forgottenrelics.co.uk/tunnels/gallery/wapping.html|title=Wapping Tunnel|access-date=30 September 2014|archive-date=12 May 2021|archive-url=https://web.archive.org/web/20210512103820/http://forgottenrelics.co.uk/tunnels/gallery/wapping.html|url-status=dead}}</ref><ref name="maund">{{cite book |title= Merseyrail electrics: the inside story |author=Maund, T.B. |publisher= NBC Books |year=2001 |location=Sheffield |oclc=655126526}}</ref>
* 1832, [[Liverpool Lime Street railway station|Lime Street railway station]] tunnel, Liverpool. A two track rail tunnel, {{convert|1.811|km|mi|abbr=on}} long was bored under the metropolis from Edge Hill in the east of the city to Lime Street in Liverpool's city centre. The tunnel was in use from 1832 being used to transport building materials to the new Lime St station while under construction. The station and tunnel was opened to passengers in 1836. In the 1880s the tunnel was converted to a deep cutting, open to the atmosphere, being four tracks wide. This is the only occurrence of a major tunnel being removed. Two short sections of the original tunnel still exist at Edge Hill station and further towards Lime Street, giving the two tunnels the distinction of being the oldest rail tunnels in the world still in use, and the oldest in use under streets.<ref>[http://www.hows.org.uk/personal/rail/incline/lls.htm Liverpool Lime St] {{webarchive |url=https://web.archive.org/web/20160304031133/http://www.hows.org.uk/personal/rail/incline/lls.htm |date=4 March 2016 }}</ref> Over time a {{convert|525|m|mi|abbr=on}} section of the deep cutting has been converted back into tunnel due to sections having buildings built over. 
* [[Box Tunnel]] in England, which opened in 1841, was the longest railway tunnel in the world at the time of construction. It was dug by hand, and has a length of {{convert|2.9|km|mi|abbr=on}}.
* The {{convert|1.1|km|mi|abbr=on}} 1842 Prince of Wales Tunnel, in [[Shildon]] near Darlington, England, is the oldest sizeable tunnel in the world still in use under a settlement.
* The [[Victoria Tunnel (Newcastle)|Victoria Tunnel Newcastle]] opened in 1842, is a {{convert|2.4|mi|km|order=flip|adj=on}} subterranean wagonway with a maximum depth of {{convert|85|ft|0|order=flip}} that drops {{convert|222|ft|0}} from entrance to exit. The tunnel runs under Newcastle upon Tyne, England, and originally exited at the River Tyne. It remains largely intact. Originally designed to carry coal from [[Spital Tongues]] to the river, in WW2 part of the tunnel was used as a shelter. Under the management of a charitable foundation called the Ouseburn Trust it is currently used for heritage tours.
* The [[Thames Tunnel]], built by [[Marc Isambard Brunel]] and his son [[Isambard Kingdom Brunel]] opened in 1843, was the first tunnel (after Terelek) traversing under a water body, and the first to be built using a [[tunnelling shield]]. Originally used as a foot-tunnel, the tunnel was converted to a railway tunnel in 1869 and was a part of the [[East London Line]] of the [[London Underground]] until 2007. It was the oldest section of the network, although not the oldest purpose built rail section. From 2010 the tunnel became a part of the [[London Overground]] network.
* The {{convert|3.34|km|mi|abbr=on}} [[Victoria Tunnel (Liverpool)|Victoria Tunnel]]/[[Waterloo Tunnel]] in [[Liverpool]], England, was bored under a metropolis opening in 1848. The tunnel was initially used only for rail freight serving the Waterloo Freight terminal, and later freight and passengers serving the [[Liverpool Riverside railway station|Liverpool ship liner terminal]]. The tunnel's path is from Edge Hill in the east of the city to the north end Liverpool docks at [[Waterloo Dock (Liverpool)|Waterloo Dock]]. The tunnel is split into two tunnels with a short open air cutting linking the two. The cutting is where the cable hauled trains from Edge Hill were hitched and unhitched. The two tunnels are effectively one on the same centre line and are regarded as one. However, as initially the {{convert|2375|m|mi|abbr=on}} long Victoria section was originally cable hauled and the shorter {{convert|862|m|yd|abbr=on}} Waterloo section was locomotive hauled, two separate names were given, the short section was named the [[Waterloo Tunnel]]. In 1895 the two tunnels were converted to locomotive haulage. Used until 1972, the tunnel is still in excellent condition. A short section of the Victoria tunnel at Edge Hill is still used for shunting trains. The tunnel is being considered for reuse by the [[Merseyrail]] network. Stations cut into the tunnel are being considered and also reuse by a monorail system from the proposed [[Liverpool Waters]] redevelopment of Liverpool's Central Docks has been proposed.<ref>{{cite web|url=http://www.forgottenrelics.co.uk/tunnels/gallery/victoria.html|title=Victoria Tunnel|access-date=30 September 2014}}</ref><ref>{{cite web|url=http://www.forgottenrelics.co.uk/tunnels/gallery/waterloo.html|title=Waterloo Tunnel|access-date=30 September 2014}}</ref>
* The summit tunnel of the [[Semmering railway]], the first Alpine tunnel, was opened in 1848 and was {{convert|1.431|km|mi|abbr=on}} long. It connected rail traffic between [[Vienna]], the capital of [[Austro-Hungarian Empire]], and [[Trieste]], its port.
* The [[Giovi Rail Tunnel]] through the [[Appennini]] Mounts opened in 1854, linking the capital city of the [[Kingdom of Sardinia]], [[Turin]], to its port, [[Genoa]]. The tunnel was {{convert|3.25|km|mi|abbr=on}} long.
* The oldest underground sections of the [[London Underground]] were built using the cut-and-cover method in the 1860s, and opened in January 1863. What are now the [[Metropolitan line|Metropolitan]], Hammersmith & City and Circle lines were the first to prove the success of a [[rapid transit|metro]] or subway system.
*On 18 June 1868, the [[Central Pacific Railroad]]'s {{convert|1,659|ft|m|order=flip|adj=on}} Summit Tunnel (Tunnel #6) at [[Donner Pass]] in the California [[Sierra Nevada (U.S.)|Sierra Nevada]] mountains was opened, permitting the establishment of the commercial mass transportation of passengers and freight over the Sierras for the first time. It remained in daily use until 1993, when the [[Southern Pacific Railroad]] closed it and transferred all rail traffic through the {{convert|10322|ft|m|adj=on|order=flip}} long Tunnel #41 (a.k.a. "The Big Hole") built a mile to the south in 1925.
* In 1870, after fourteen years of works, the [[Fréjus Rail Tunnel]] was completed between France and Italy, being the second-oldest Alpine tunnel, {{convert|13.7|km|mi|abbr=on}} long. At that time it was the longest in the world.
* The third Alpine tunnel, the [[Gotthard Rail Tunnel]], between northern and southern Switzerland, opened in 1882 and was the longest rail tunnel in the world, measuring {{convert|15|km|mi|abbr=on}}.
* The 1882 [[Col de Tende Road Tunnel]], at {{convert|3.182|km|mi|abbr=on}} long, was one of the first long road tunnels under a pass, running between France and Italy.
* The [[Mersey Railway]] tunnel opened in 1886, running from Liverpool to Birkenhead under the River Mersey. The [[Mersey Railway]] was the world's first deep-level underground railway. By 1892 the extensions on land from Birkenhead Park station to Liverpool Central Low level station gave a tunnel {{convert|3.12|mi|km|abbr=on|order=flip}} in length. The under river section is {{convert|0.75|mi|km|abbr=on|order=flip}} in length, and was the longest underwater tunnel in world in January 1886.<ref>{{cite web|url=http://www.upton-wirral.co.uk/wir2.php|title=Mersey Railway Tunnel|access-date=30 September 2014|url-status=dead|archive-url=https://web.archive.org/web/20130529074903/http://www.upton-wirral.co.uk/wir2.php|archive-date=29 May 2013}}</ref><ref>[http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=1151 Engineering Timelines – Mersey Railway] {{webarchive |url=https://web.archive.org/web/20120322113859/http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=1151 |date=22 March 2012 }}</ref>
* The rail [[Severn Tunnel]] was opened in late 1886, at {{convert|7.008|km|mi|abbr=on}} long, although only {{convert|3.62|km|mi|abbr=on}} of the tunnel is actually under the River Severn. The tunnel replaced the Mersey Railway tunnel's longest under water record, which was held for less than a year.
* [[James Greathead]], in constructing the City & South London Railway tunnel beneath the Thames, opened in 1890, brought together three key elements of tunnel construction under water: 
*#shield method of excavation; 
*#permanent cast iron tunnel lining; 
*#construction in a compressed air environment to inhibit water flowing through soft ground material into the tunnel heading.<ref name="nrhp_stclair"/>
* Built in sections between 1890 and 1939, the section of London Underground's [[Northern line]] from Morden to East Finchley via Bank was the longest railway tunnel in the world at {{convert|27.8|km|mi|abbr=on}} in length.
* [[St. Clair Tunnel]], also opened later in 1890, linked the elements of the Greathead tunnels on a larger scale.<ref name="nrhp_stclair">{{Cite web |last=Lange |first=Robie S. |date=February 1993 |title=National Register of Historic Places Inventory-Nomination: St. Clair River Tunnel / St. Clair Railroad Tunnel |url={{NHLS url|id=70000684}} |access-date=2024-01-23 |website=National Park Service}}</ref>
* In 1906 the fourth Alpine tunnel opened, the [[Simplon Tunnel]], between Switzerland and Italy. It is {{convert|19.8|km|mi|abbr=on}} long, and was the longest tunnel in the world until 1982. It was also the deepest tunnel in the world, with a maximum rock overlay of approximately {{convert|2150|m|abbr=on}}.
* The 1927 [[Holland Tunnel]] was the first underwater tunnel designed for automobiles. The construction required a novel [[Holland Tunnel#Ventilation system|ventilation system]].
* In 1945 the [[Delaware Aqueduct]] tunnel was completed, supplying water to New York City. At {{convert|137|km|mi|abbr=on}} it is the longest tunnel in the world.
* In 1988 the {{convert|53.850|km|mi|abbr=on}} long [[Seikan Tunnel]] in Japan was completed under the [[Tsugaru Strait]], linking the islands of [[Honshu]] and [[Hokkaido]]. It was the longest railway tunnel in the world at that time.
* [[Ryfast]] is the longest undersea road tunnel. It is {{cvt|14.3|km|mi}} in length.<ref>{{cite web|url=https://www.tu.no/artikler/i-dag-sprenges-siste-fjellrest-vekk-i-verdens-lengste-undersjoiske-veitunnel/410538|title=Nå er siste fjellrest sprengt vekk i verdens lengste undersjøiske veitunnel|date=26 October 2017}}</ref> The tunnel opened for use in 2020.

=== Longest ===
{{Main|List of longest tunnels}}
[[File:Gotthard Base Tunnel.jpg|thumb|The Gotthard Base Tunnel is the first flat route through a major mountain range.]]
* The [[Thirlmere Aqueduct]] in [[North West England]], United Kingdom is sometimes considered the longest tunnel, of any type, in the world at {{convert|154|km|mi|abbr=on}}, though the aqueduct's tunnel section is not continuous.{{dubious|date=March 2014}}
* The [[Dahuofang Water Tunnel]] in [[China]], opened in 2009, is the third longest water tunnel in the world at {{cvt|85.3|km}} length. 
* The [[Gotthard Base Tunnel]] in [[Switzerland]], opened in 2016, is the longest and deepest railway tunnel in the world at {{cvt|57.1|km}} length and {{cvt|2450|m}} maximum depth below the [[Saint-Gotthard Massif|Gotthard Massif]]. It provides a [[NRLA|flat transit route]] between the North and South of Europe under the [[Swiss Alps]], at a maximum elevation of {{cvt|549|m}}. 
* The [[Seikan Tunnel]] in [[Japan]] connects the main island of [[Honshu]] with the northern island of [[Hokkaido]] by rail. It is {{convert|53.9|km|adj=on}} long, of which {{cvt|23.3|km}} are crossing the [[Tsugaru Strait]] undersea.
* The [[Channel Tunnel]] crosses the [[English Channel]] between [[France]] and the [[United Kingdom]]. It has a total length of {{cvt|50|km}}, of which {{cvt|39|km}} are the world's longest undersea tunnel section.
* The [[Lötschberg Base Tunnel]] in [[Switzerland]] was the longest land rail tunnel, with a length of {{cvt|34.5|km}}, from its inauguration in 2007 until the completion of the [[Gotthard Base Tunnel]] in 2016.
* The [[Lærdal Tunnel]] in Norway from [[Lærdal]] to [[Aurland]] is the world's longest road tunnel, intended for cars and similar vehicles, at {{convert|24.5|km|mi|abbr=on}}.
* The [[Zhongnanshan Tunnel]] in [[People's Republic of China]] opened in January 2007 is the world's second longest [[highway]] tunnel and the longest mountain road tunnel in Asia, at {{convert|18|km|mi|abbr=on}}.
<!-- construction not even started*The [[Ryfast]] road program in [[Stavanger]], Norway, includes the tunnel [[Solbakktunnelen]], which is scheduled to be opened within 2015. This tunnel will be {{convert|14|km|mi|abbr=on}} long, making it both the world's longest underwater road tunnel and longest underwater highway tunnel. The tunnel will have four driving lanes in total, and a speed limit of {{convert|90|km/h|mph|abbr=on|lk=on}}. -->
* The longest canal tunnel is the [[Rove Tunnel]] in France, over {{convert|7.12|km|mi|abbr=on}} long.

=== Notable ===
<!-- NOTE TO EDITORS: Sort entries by earliest known or expected date of completion. List distances by kilometres first, using the template {{convert|0.00|km|mi|abbr=on}} -->
[[File:Tunnel-large.jpg|thumb|right|The [[Big Dig]] road vehicle tunnel in [[Boston]], U.S.]]
[[File:Gerrards Cross railway station 2.jpg|thumb|right|The [[Gerrards Cross tunnel]] in England, completed in 2010. Looking west towards the station in March 2005, showing the extent of construction three months before a small section collapsed.]]
[[File:Sideling Hill Tunnel 2009.jpg|thumb|right|The eastern portal of the abandoned [[Sideling Hill Tunnel]], Pennsylvania, U.S., in 2009]]
* The [[Moffat Tunnel]], opened in 1928, passes under the [[Continental Divide of the Americas]] in [[Colorado]]. The tunnel is {{convert|10.0|km|mi|abbr=on}} long and at an elevation of {{convert|2816|m|ft|abbr=on}} is the highest active railroad tunnel in the U.S. (The inactive [[Tennessee Pass Line]] and the historic [[Alpine Tunnel]] are higher.)
* [[Williamson's tunnels]] in [[Liverpool]], from 1804 and completed around 1840 by a wealthy eccentric, are probably the largest underground [[folly]] in the world. The tunnels were built with no functional purpose. 
* The [[Chicago Tunnel Company|Chicago freight tunnel network]] is the largest urban street tunnel network, comprising {{convert|97|km|mi|abbr=on}} of tunnels beneath the majority of downtown [[Chicago]] streets. It operated between 1906 and 1956 as a freight network, connecting building basements and railway stations. Following a [[Chicago flood|1992 flood]] the network was sealed, although some parts still carry utility and communications infrastructure.
* The [[Pennsylvania Turnpike]] opened in 1940 with seven tunnels, most of which were bored as part of the stillborn [[South Pennsylvania Railroad]] and giving the highway the nickname "Tunnel Highway". Four of the tunnels ([[Allegheny Mountain Tunnel|Allegheny Mountain]], [[Tuscarora Mountain Tunnel|Tuscarora Mountain]], [[Kittatinny Mountain Tunnel|Kittatinny Mountain]], and [[Blue Mountain Tunnel|Blue Mountain]]) remain in active use, while the other three ([[Laurel Hill Tunnel|Laurel Hill]], [[Rays Hill Tunnel|Rays Hill]], and [[Sideling Hill Tunnel|Sideling Hill]]) were bypassed in the 1960s; the latter two tunnels are on a bypassed section of the Turnpike now commonly known as the [[Abandoned Pennsylvania Turnpike]].
* The [[Essingeleden|Fredhälls road tunnel]] was opened in 1966, in [[Stockholm, Sweden]], and the [[Elbe Tunnel (1975)|New Elbe road tunnel]] opened in 1975 in [[Hamburg, Germany]]. Both tunnels handle around 150,000 vehicles a day, making them two of the most trafficked tunnels in the world.
* The [[Honningsvåg Tunnel]] ({{convert|4.443|km|mi|abbr=on|sigfig=3}} long) opened in 1999 on [[European route E69]] in Norway as the world's northernmost road tunnel, except for mines (which exist on [[Svalbard]]).
* The Central Artery road tunnel in [[Boston, Massachusetts]], is a part of the larger [[Big Dig (Boston, Massachusetts)|Big Dig]] completed around 2007, and carries approximately 200,000 vehicles/day under the city along [[Interstate 93]], [[U.S. Route 1 in Massachusetts|US Route 1]], and [[Massachusetts Route 3]], which share a [[Concurrency (road)|concurrency]] through the tunnels. The Big Dig replaced Boston's old badly deteriorated I-93 elevated highway.
* The Stormwater Management And Road Tunnel or [[SMART Tunnel]], is a combined storm drainage and road structure opened in 2007 in [[Kuala Lumpur]], Malaysia. The {{convert|9.7|km|mi|abbr=on}} tunnel is the longest stormwater [[drainage tunnel]] in South East Asia and second longest in Asia. The facility can be operated as a simultaneous traffic and stormwater passage, or dedicated exclusively to stormwater when necessary.
* The [[Eiksund Tunnel]]<ref>{{Cite web |date=2008-03-02 |title=Safety: World's Deepest Subsea Tunnel Opens in Norway |url=http://www.tunnelintelligence.com/safety-in-detail-106.html |url-status=usurped |archive-url=https://web.archive.org/web/20120317104705/http://www.tunnelintelligence.com/safety-in-detail-106.html |archive-date=2012-03-17 |access-date=2024-01-23 |website=www.tunnelintelligence.com}}</ref> on national road Rv 653 in Norway is the world's deepest subsea road tunnel, measuring {{convert|7.776|km|mi|abbr=on}} long, with deepest point at {{convert|-287|m|ft|abbr=on}} below the sea level, opened in February 2008.
* [[Gerrards Cross Tunnel|Gerrards Cross railway tunnel]], in England, opened in 2010, is notable in that it converted an existing railway cutting into a tunnel to create ground to build a supermarket over the tunnel. The railway in the cutting was first opened around 1906, stretching over 104 years to complete a railway tunnel. The tunnel was built using the cover method with craned in prefabricated forms in order to keep the busy railway operating. A branch of the [[Tesco]] supermarket chain occupies the newly created ground above the railway tunnel, with an adjacent existing railway station at the end of the tunnel. During construction, a portion of the tunnel collapsed when soil cover was added. The prefabricated forms were covered with a layer of reinforced concrete after the collapse.<ref>{{cite web|url=http://www.nce.co.uk/news/transport/costain-finishes-gerrards-cross-tunnel-rebuild/5218141.article|title=Costain finishes Gerrards Cross tunnel rebuild|access-date=30 September 2014|date=19 May 2010}}</ref>
* The [[Fenghuoshan tunnel]], completed in 2005 on the [[Qinghai-Tibet railway]] is the world's highest railway tunnel, about {{convert|4.905|km|mi|abbr=on|sigfig=3}} above sea level and {{convert|1,338|m|mi|abbr=on|sigfig=3}} long.
* The [[La Línea (Road Pass)|La Linea]] Tunnel in [[Colombia]], 2016, is the longest, {{convert|8.58|km|mi|abbr=on|sigfig=3}}, mountain tunnel in South America. It crosses beneath a mountain at {{convert|2500|m|ft|abbr=on|sigfig=5}} <!-- needs clarification --> above sea level with six traffic lanes, and it has a parallel emergency tunnel. The tunnel is subject to serious [[groundwater]] pressure. The tunnel will link [[Bogotá]] and its urban area with the coffee-growing region, and with the main port on the Colombian Pacific coast.
* The [[Tunnel and Reservoir Plan|Chicago Deep Tunnel Project]] is a network of {{convert|175|km|mi|abbr=on}} of [[drainage tunnel]]s designed to reduce flooding in the [[Chicago metropolitan area|Chicago area]]. Started in the mid-1970s, the project is due to be completed in 2029.
* [[New York City Water Tunnel No. 3]], started in 1970, has an expected completion beyond 2026,<ref>{{Cite web |last=Kensinger |first=Nathan |date=22 April 2021 |title=NYC's Giant Water Tunnel Begins Work On Final Shafts, Following 50 Years Of Construction |url=https://gothamist.com/news/nycs-giant-water-tunnel-begins-work-on-final-shafts-following-50-years-of-construction |url-access=registration |access-date=15 September 2022 |website=Gothamist |language=en |quote=These last two shafts are now expected to be done by 2026, according to the DEP, but the tunnel still won't be complete. The original plans called for one more extension—a 14-mile conduit between Yonkers, the Bronx and Queens.}}</ref> and will measure more than {{convert|97|km|mi|abbr=on}} long.<ref>{{cite web|url=http://www.nyc.gov/html/dep/html/news/3rdtunnel.html |archive-url=https://web.archive.org/web/20070621120811/http://www.nyc.gov/html/dep/html/news/3rdtunnel.html |archive-date=21 June 2007 |title=City Water Tunnel No. 3 |access-date=19 April 2013}}</ref>

== Mining ==
{{main|Mining}}
[[File:New Pinghsi Coal Tunnel.JPG|thumb|Tunnel formerly used for [[Taiwan Coal Mine Museum|coal mining]] in [[New Taipei]], [[Taiwan]]]]
The use of tunnels for mining is called [[drift mining]]. Drift mining can help find coal, goal, iron, and other minerals, just like normal mining.

Sub-surface mining consists of digging tunnels or shafts into the earth to reach buried ore deposits.

== Military use ==
{{see also|Sapper}}
Some tunnels are not for transport at all but rather, are fortifications, for example [[Mittelwerk]] and [[Cheyenne Mountain Complex]]. Excavation techniques, as well as the construction of underground bunkers and other habitable areas, are often associated with [[tunnel warfare|military use during armed conflict]], or civilian responses to threat of attack. Another use for tunnels was for the storage of chemical weapons<ref>{{cite web|url=https://www.youtube.com/watch?v=kp1EFlngjKQ | archive-url=https://web.archive.org/web/20110517201322/http://www.youtube.com/watch?v=kp1EFlngjKQ| archive-date=17 May 2011|title=Glenbrook Tunnel – Alcatraz Down Under – History Channel |publisher=Youtube.com |access-date=19 April 2013}}</ref><ref>[http://bluemountains.yourguide.com.au/news/local/news/general/author-lifts-lid-on-chemical-wartime-history/307763.aspx Author lifts lid on chemical wartime history – Local News – News – General – Blue Mountains Gazette] {{webarchive |url=https://web.archive.org/web/20090109001520/http://bluemountains.yourguide.com.au/news/local/news/general/author-lifts-lid-on-chemical-wartime-history/307763.aspx |date=9 January 2009 }}</ref> [http://www.mustardgas.org/].

== Secret tunnels ==
{{main|Secret passage|Smuggling tunnel}}
[[File:Underground Railroad Tunnel Door.jpg|thumb|upright|Door to a compartment where runaway slaves would sleep, on the [[Underground Railroad]]]]
[[secret passage|Secret tunnels]] have given entrance to or escape from an area, such as the [[Cu Chi Tunnels]] or the [[Gaza Strip smuggling tunnels|smuggling tunnels in the Gaza Strip]] which connect it to [[Egypt]]. Although the [[Underground Railroad]] network used to transport escaped slaves was "underground" mostly in the sense of secrecy, hidden tunnels were occasionally used. Secret tunnels were also used during the [[Cold War]], under the [[Berlin Wall]] and elsewhere, to smuggle refugees, and for [[espionage]].

[[Smuggler]]s use secret tunnels to transport or store [[contraband]], such as [[illegal drugs]] and [[weapon]]s. Elaborately engineered {{convert|1000|ft|m|adj=on|order=flip}} tunnels built to smuggle drugs across the Mexico-US border were estimated to require up to 9 months to complete, and an expenditure of up to $1 million.<ref name=Audi>{{cite news|last1=Audi|first1=Tamara|title=Drug Tunnels Have Feds Digging for Answers|url=https://online.wsj.com/news/articles/SB10001424127887323854904578264382749570570|access-date=4 October 2014|work=The Wall Street Journal|date=31 January 2013}}</ref> Some of these tunnels were equipped with lighting, ventilation, telephones, drainage pumps, hydraulic elevators, and in at least one instance, an electrified rail transport system.<ref name="Audi" /> Secret tunnels have also been used by thieves to break into bank vaults and retail stores after hours.<ref name=Colchester>{{cite news|last1=Colchester|first1=Max|title=Thieves Drill Into Paris Bank Vault|url=https://online.wsj.com/articles/SB10001424052702304739104575153530631794248|access-date=4 October 2014|work=The Wall Street Journal|date=31 March 2010}}</ref><ref name=Evans>{{cite news|last1=Evans|first1=Peter|title=Where 'Criminal Underworld' Is More Than a Euphemism|url=https://online.wsj.com/articles/where-criminal-underworld-is-more-than-a-euphemism-1412383000|access-date=4 October 2014|work=The Wall Street Journal|date=3 October 2014}}</ref> Several tunnels have been discovered by the [[Border Security Forces]] across the [[Line of Control]] along the [[India-Pakistan border]], mainly to allow terrorists access to the Indian territory of [[Jammu and Kashmir (union territory)|Jammu and Kashmir]].<ref name="arniatunnel">{{cite news|last1=Khajuria|first1=Ravi Krishnan|title=Day after India-Pakistan flag meet, BSF detects trans-border tunnel in Jammu's Arnia sub-sector|url=http://www.hindustantimes.com/india-news/under-construction-tunnel-being-dug-from-pakistan-side-unearthed-by-bsf-in-jammu/story-B1qgulsJCMQ8FOxWs3LYyO.html|access-date=10 December 2017 |work=[[Hindustan Times]]|publisher=1 October 2017}}</ref><ref name="sambhatunnel">{{cite news|last1=Iqbal|first1=Sheikh Zaffar|title=20-Foot Tunnel From Pakistan Found By BSF At Sambha, Jammu and Kashmir|url=https://www.ndtv.com/india-news/20-foot-tunnel-from-pakistan-found-by-bsf-at-sambha-jammu-and-kashmir-1659393|access-date=10 December 2017|work=[[NDTV]]|date=14 February 2017}}</ref>

[[Metro-2]] is a clandestine deep underground metro system in the [[Moscow]] metropolitan area. It was designed to provide the [[Politics of the Soviet Union|Soviet leadership]] with secure wartime [[Emergency evacuation|evacuation]] routes, [[Communications protection|communication hubs]], and [[Command and control#Command and control centers|command posts]], including a dedicated [[bunker]] for the [[National Command Authority (United States)|national command authority]].

The actual usage of [[erdstall]] tunnels is unknown but theories connect it to a rebirth ritual.

== Natural tunnels ==
[[File:Yongmun-gul at Daedunsan.jpg|thumb|upright|View through a natural tunnel in South Korea]]
* [[Lava tube]]s are emptied lava conduits, formed during volcanic eruptions by flowing and cooling lava.
* [[Natural Tunnel State Park]] (Virginia, US) features an {{convert|850|ft|m|adj=on|order=flip}} natural tunnel, really a limestone [[cave]], that has been used as a railroad tunnel since 1890.
* [[Punarjani Guha]] in [[Kerala, India]]. Hindus believe that crawling through the tunnel (which they believe was created by a Hindu god) from one end to the other will wash away all of one's sins and thus allow one to attain rebirth. Only men are permitted to crawl through the tunnel.
* [[Torghatten]], a Norwegian island with a hat-shaped silhouette, has a natural tunnel in the middle of the hat, letting light come through. The {{convert|160|m|ft|adj=on}} long, {{convert|35|m|ft|adj=on}} high, and {{convert|20|m|ft|adj=on}} wide tunnel is said to be the hole made by an arrow of the angry [[troll]] Hestmannen, the hill being the hat of the troll-king of [[Sømna Municipality|Sømna]] trying to save the beautiful [[Lekamøya]]. The tunnel is thought actually to be the work of ice. The sun shines through the tunnel during two few minutes long periods every year.<ref>{{cite news |last1=Warholm |first1=Harald |date=10 November 2014 |title=Hobbyfotografen har ventet tre år på dette sjeldne blinkskuddet |trans-title=The hobby photographer has waited three years for this rare shot |url=http://www.nrk.no/nordland/jubler-over-sjeldent-blinkskudd-av-fjellet-torghatten-1.12034377 |access-date=13 November 2014 |work=nrk.no |language=Norwegian}}</ref>

== Major accidents ==
* [[Clayton Tunnel rail crash]] (1861) – confusion about block signals leading to collision, 23 killed.
* [[Welwyn Tunnel rail crash]] (1866) – train failed in tunnel, guard did not protect train.
* [[Paris Métro train fire]] (1904) – train fire in Couronnes underground station, 84 killed by smoke and gases.
* [[Church Hill Tunnel|Church Hill Tunnel collapse]] (1925) – tunnel collapse on a work train during renovation, killing four men and trapping a steam locomotive and ten flat cars.
* [[Balvano train disaster]] (1944) – asphyxiation of about 500 "unofficial" passengers on freight train.
* [[Caldecott Tunnel fire]] (1982) – major motor vehicle tunnel crash and fire.
* [[1996 Channel Tunnel fire|Channel Tunnel fire]] (1996) – Train carrying [[Large goods vehicle|Heavy Good Vehicles (HGV)]] caught on fire. 
* [[Death of Diana, Princess of Wales|Princess Diana's death]] (1997) – Car crash in [[Pont de l'Alma]] tunnel, [[Paris]], which killed [[Princess Diana]].
* [[Mont Blanc tunnel fire|Mont Blanc Tunnel fire]] (1999) – Transport truck caught on fire and combusted inside tunnel.
* [[Big Dig ceiling collapse|Big Dig Ceiling collapse]] (2006) – Concrete ceiling panel falls in [[Fort Point Tunnel|Fort Point]] tunnel, [[Boston]], which causes the [[Big Dig|Big Dig project]] to be closed for a year.

== See also ==
{{div col}}
*[[Euphrates Tunnel]]
*[[Cattle creep]]
*[[Counter-beam lighting]]
*[[Culvert]]
*[[Hobby tunneling]]
*[[Megaproject]]
* [[Rapid transit]]
*[[Sequential Excavation Method]]
*[[Structure gauge]] – measure of maximum physical clearance in a tunnel
*[[Tree tunnel]] – tunnel-like effect from tree canopies above a road
*[[Tunnel tree]] – tunnel bored through the trunk of a tree
*[[Tunnels in popular culture]]
* [[Underground living]]

{{div col end}}

== References ==
{{Reflist|30em}}

== Bibliography ==
* {{cite book
|last      = Ellis
|first     = Iain W
|year      = 2015
|title     = Ellis' British Railway Engineering Encyclopaedia
|publisher = Lulu.com
|edition   = 3rd Revised
|isbn      = 978-1-326-01063-8}}
* Railway Tunnels in Queensland by Brian Webber, 1997, {{ISBN|0-909937-33-8}}.
* Sullivan, Walter. [https://www.nytimes.com/1986/06/24/science/progress-in-technology-revives-interest-in-great-tunnels.html?&pagewanted=all Progress In Technology Revives Interest In Great Tunnels], ''[[The New York Times]]'', 24 June 1986. Retrieved 15 August 2010.

== External links ==
{{AmCyc Poster}}
{{commons category|Tunnels}}
* [http://www.ita-aites.org/ ITA-AITES International Tunnelling Association]
* [http://www.tunnelsonline.info/ Tunnels & Tunnelling International magazine]

{{Infrastructure}}
{{Geotechnical engineering}}
{{Portal bar|Architecture}}

{{Man-made and man-related Subterranea}}
{{Authority control}}

[[Category:Tunnels| ]]
[[Category:Crossings]]
[[Category:Civil engineering]]
[[Category:Transport buildings and structures]]
[[Category:Earthworks (engineering)]]
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