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{{Short description|Spring produced by the emergence of geothermally heated groundwater}}
{{Redirect|Hot springs|other uses|Hot Springs (disambiguation)}}
[[File:Aerial view of Grand Prismatic (23428929375).jpg|thumb|upright=1.1|[[Grand Prismatic Spring]] and Midway Geyser Basin in [[Yellowstone National Park]]]]

A '''hot spring''', '''thermal spring''', '''hydrothermal spring''', or '''geothermal spring''' is a [[Spring (hydrology)|spring]] produced by the emergence of [[Geothermal activity|geothermally heated]] [[groundwater]] onto the surface of the Earth. The groundwater is heated either by shallow bodies of [[magma]] (molten rock) or by circulation through [[fault (geology)|faults]] to hot rock deep in the [[Earth's crust]].

Hot spring water often contains large amounts of dissolved minerals. The chemistry of hot springs ranges from acid sulfate springs with a [[pH]] as low as 0.8, to alkaline chloride springs saturated with [[silica]], to bicarbonate springs saturated with [[carbon dioxide]] and [[carbonate minerals]]. Some springs also contain abundant dissolved iron. The minerals brought to the surface in hot springs often feed communities of [[extremophiles]], microorganisms adapted to extreme conditions, and it is possible that life on Earth had its origin in hot springs.<ref>{{cite journal|last1=Farmer |first1=J.D. |year=2000 |title=Hydrothermal systems: doorways to early biosphere evolution |journal=GSA Today |volume=10 |number=7 |pages=1–9 |url=https://www.geosociety.org/gsatoday/archive/10/7/pdf/gt0007.pdf |access-date=25 June 2021}}</ref><ref>{{cite journal |last1=Des Marais |first1=David J. |last2=Walter |first2=Malcolm R. |title=Terrestrial Hot Spring Systems: Introduction |journal=Astrobiology |date=2019-12-01 |volume=19 |issue=12 |pages=1419–1432 |doi=10.1089/ast.2018.1976|pmid=31424278 |pmc=6918855 |bibcode=2019AsBio..19.1419D }}</ref>

Humans have made use of hot springs for bathing, relaxation, or medical therapy for thousands of years. However, some are hot enough that immersion can be harmful, leading to scalding and, potentially, death.<ref name=":0">{{Cite web|title=Hot Springs/Geothermal Features - Geology (U.S. National Park Service)|url=https://www.nps.gov/subjects/geology/hot-springs.htm|access-date=2021-02-11|website=www.nps.gov|language=en}}</ref>

==Definitions==
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There is no universally accepted definition of a hot spring. For example, one can find the phrase ''hot spring'' defined as
* any spring heated by [[geothermal activity]]<ref>{{cite web|title=MSN Encarta definition of hot spring |url=http://encarta.msn.com/encnet/features/dictionary/DictionaryResults.aspx?refid=1861692101 |archive-url=https://web.archive.org/web/20090122234830/http://encarta.msn.com/encnet/features/dictionary/DictionaryResults.aspx?refid=1861692101 |archive-date=2009-01-22 }}</ref>
* a spring with water temperatures above its surroundings<ref>[http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=hot+spring Miriam-Webster Online dictionary definition of hot spring]</ref><ref>[http://www.bartleby.com/65/ho/hotsprin.html Columbia Encyclopedia, sixth edition, article on hot spring] {{webarchive|url=https://web.archive.org/web/20070211173244/http://www.bartleby.com/65/ho/hotsprin.html |date=2007-02-11 }}</ref>
* a natural spring with water temperature above [[human body temperature]] (normally about {{convert|37|°C|°F}})<ref>[http://www.wordsmyth.net/live/home.php?script=search&matchent=hot+spring&matchtype=exact Wordsmyth definition of hot spring]</ref><ref>[http://www.bartleby.com/61/35/H0293500.html American Heritage dictionary, fourth edition (2000) definition of hot spring] {{webarchive|url=https://web.archive.org/web/20070310162547/http://www.bartleby.com/61/35/H0293500.html |date=2007-03-10 }}</ref><ref name="pubs.nrc-cnrc.gc.ca">{{cite journal|doi=10.1139/e03-083 |author1=Allan Pentecost |author2=B. Jones |author3=R.W. Renaut |title=What is a hot spring? |journal=Can. J. Earth Sci. |volume=40 |issue=11 |pages=1443–6 |year=2003 |url=http://pubs.nrc-cnrc.gc.ca/cgi-bin/rp/rp2_abst_e?cjes_e03-083_40_ns_nf_cjes |bibcode=2003CaJES..40.1443P |archive-url=https://web.archive.org/web/20070311064433/http://pubs.nrc-cnrc.gc.ca/cgi-bin/rp/rp2_abst_e?cjes_e03-083_40_ns_nf_cjes |archive-date=2007-03-11 |url-access=subscription }} provides a critical discussion of the definition of a hot spring.</ref><ref>[http://www.infoplease.com/dictionary/hot+spring Infoplease definition of hot spring]</ref><ref>[http://dictionary.reference.com/search?q=hot+spring&r=66 Random House Unabridged Dictionary, © Random House, Inc. 2006. definition of hot spring]</ref>
[[File:Rio Quente 16 (27822967745).jpg|thumb|Hot water springs in [[Rio Quente|Rio Quente, Brazil]]]]
* a natural spring of water whose temperature is greater than {{convert|21|°C|°F|0}}<ref>[http://dictionary.reference.com/search?q=hot+spring&r=66 Wordnet 2.0 definition of hot spring]</ref><ref>[http://www.ultralingua.com/onlinedictionary/?service=ee&text=hot+spring Ultralingua Online Dictionary definition of hot spring]</ref><ref>[http://www.rhymezone.com/r/rhyme.cgi?Word=hot_spring Rhymezone definition of hot spring]</ref><ref>[http://lookwayup.com/lwu.exe/lwu/d?s=f&w=hot_spring Lookwayup definition of hot spring]</ref>
* a type of thermal spring whose water temperature is usually {{convert|6 to 8|C-change|0|abbr=on}} or more above mean air temperature.<ref>{{cite book |author=Don L. Leet |title=Physical Geology |publisher=Prentice-Hall |location=Englewood Cliffs, NJ |year=1982 |edition=6th |url=http://www.webref.org/geology/h/hot_spring.htm |quote=A thermal spring is defined as a spring that brings warm or hot water to the surface. |isbn=978-0-13-669706-0 |access-date=2006-11-03 |archive-date=2010-10-02 |archive-url=https://web.archive.org/web/20101002101223/http://www.webref.org/geology/h/hot_spring.htm }} Leet states that there are two types of thermal springs; hot springs and warm springs. Note that by this definition, "thermal spring" is not synonymous with the term "hot spring".</ref>
* a spring with water temperatures above {{convert|50|°C|°F}}<ref>US [[NOAA]] Geophysical Data Center definition</ref>

The related term "'''warm spring'''" is defined as a spring with water temperature less than a hot spring by many sources, although Pentecost et al. (2003) suggest that the phrase "warm spring" is not useful and should be avoided. In 1923, Menzier proposed that a warm spring be defined as a thermal spring where the water is below that of the human body, but above that of the mean air temperature around the spring, though this definition is contested.<ref name="Out of ground-water hydrology">{{Cite tech report |last=Meinzer |first=Oscar Edward |title=Outline of ground-water hydrology, with definitions |type=Water Supply Paper |publisher=United States Geological Survey |doi=10.3133/wsp494 |year=1923 |volume=494 |page=54 |url=https://pubs.usgs.gov/publication/wsp494 |access-date=13 March 2025}}</ref><ref name="pubs.nrc-cnrc.gc.ca" />

==Sources of heat==
Water issuing from a hot spring is heated [[Geothermal energy|geothermally]], that is, with [[heat]] produced from the [[Earth's mantle]]. This takes place in two ways. In areas of high volcanic activity, [[magma]] (molten rock) may be present at shallow depths in the [[Earth's crust]]. [[Groundwater]] is heated by these shallow magma bodies and rises to the surface to emerge at a hot spring. However, even in areas that do not experience volcanic activity, the temperature of rocks within the earth increases with depth. The rate of temperature increase with depth is known as the [[geothermal gradient]]. If water percolates deeply enough into the crust, it will be heated as it comes into contact with hot rock. This generally takes place along [[Fault (geology)|faults]], where shattered rock beds provide easy paths for water to circulate to greater depths.<ref>{{cite book |last1=Macdonald |first1=Gordon A. |author-link=Gordon A. Macdonald |last2=Abbott |first2=Agatin T. |last3=Peterson |first3=Frank L. |title=Volcanoes in the sea: the geology of Hawaii |date=1983 |publisher=University of Hawaii Press |location=Honolulu |isbn=0-8248-0832-0 |edition=2nd}}</ref>

Much of the heat is created by [[radioactive decay|decay]] of naturally radioactive elements. An estimated 45 to 90 percent of the heat escaping from the Earth originates from radioactive decay of elements mainly located in the mantle.<ref name="turcotte">{{cite book | last=Turcotte | first=DL |author-link=Donald L. Turcotte |author2=Schubert, G | title=Geodynamics | publisher=Cambridge University Press | location=Cambridge, England, UK| date=2002 | edition=2nd | pages=136–7 | chapter=4 | isbn=978-0-521-66624-4 }}</ref><ref name='Anuta2006'>{{cite news | first=Joe | last=Anuta | title=Probing Question: What heats the earth's core? | date=2006-03-30 | publisher=physorg.com | url=http://www.physorg.com/news62952904.html | access-date = 2007-09-19 }}</ref><ref name=physicsworld>{{cite web|last=Johnston|first=Hamish|title=Radioactive decay accounts for half of Earth's heat|url=http://physicsworld.com/cws/article/news/2011/jul/19/radioactive-decay-accounts-for-half-of-earths-heat|work=PhysicsWorld.com|publisher=Institute of Physics|access-date=18 June 2013|date=19 July 2011}}</ref> The major heat-producing isotopes in the Earth are [[potassium-40]], [[uranium-238]], [[uranium-235]], and [[thorium-232]].<ref>{{cite news | first=Robert | last=Sanders | title=Radioactive potassium may be major heat source in Earth's core | publisher=UC Berkeley News | date=2003-12-10 | url=http://www.berkeley.edu/news/media/releases/2003/12/10_heat.shtml | access-date=2007-02-28 }}</ref> In areas with no volcanic activity, this heat flows through the crust by a slow process of [[thermal conduction]], but in volcanic areas, the heat is carried to the surface more rapidly by bodies of magma.<ref>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=978-0-521-88006-0 |edition=2nd |pages=6–13}}</ref>
[[File:Evolution of Earth's radiogenic heat.svg|thumb|upright=1.25|The [[radiogenic heat]] from the decay of <sup>238</sup>U and <sup>232</sup>Th are now the major contributors to the [[earth's internal heat budget]].]]

A hot spring that periodically jets water and steam is called a [[geyser]]. In active volcanic zones such as [[Yellowstone National Park]], magma may be present at shallow depths. If a hot spring is connected to a large natural cistern close to such a magma body, the magma may [[superheating|superheat]] the water in the cistern, raising its temperature above the normal boiling point. The water will not immediately boil, because the weight of the water column above the cistern pressurizes the cistern and suppresses boiling. However, as the superheated water expands, some of the water will emerge at the surface, reducing pressure in the cistern. This allows some of the water in the cistern to flash into steam, which forces more water out of the hot spring. This leads to a runaway condition in which a sizable amount of water and steam are forcibly ejected from the hot spring as the cistern is emptied. The cistern then refills with cooler water, and the cycle repeats.{{sfn|Macdonald|Abbott|Peterson|1983}}<ref>{{cite web |title=Hot Springs/Geothermal Features |url=https://www.nps.gov/subjects/geology/hot-springs.htm |website=Geology |publisher=National Park Service |access-date=25 June 2021 |date=10 February 2020 |ref={{harvid|National Park Service|2020}}}}</ref>

Geysers require both a natural cistern and an abundant source of cooler water to refill the cistern after each eruption of the geyser. If the water supply is less abundant, so that the water is boiled as fast as it can accumulate and only reaches the surface in the form of [[steam]], the result is a [[fumarole]]. If the water is mixed with mud and [[clay]], the result is a [[mud pot]].{{sfn|Macdonald|Abbott|Peterson|1983}}{{sfn|National Park Service|2020}}

An example of a non-volcanic warm spring is [[Warm Springs, Georgia]] (frequented for its [[physical therapy|therapeutic]] effects by [[paraplegic]] [[President of the United States|U.S. President]] [[Franklin Delano Roosevelt|Franklin D. Roosevelt]], who built the [[Little White House]] there). Here the groundwater originates as rain and snow ([[meteoric water]]) falling on the nearby mountains, which penetrates a particular [[Formation (geology)|formation]] ([[Hollis Quartzite]]) to a depth of {{convert|3000|ft||}} and is heated by the normal geothermal gradient.<ref>{{cite journal |last1=Hewett |first1=D.F. |author-link=Donnel Foster Hewett |last2=Crickmay |first2=G.W. |title=The warm springs of Georgia, their geologic relations and origin, a summary report |journal=United States Geological Survey Water Supply Paper |date=1937 |volume=819 |page=5 |doi=10.3133/wsp819|doi-access=free |bibcode=1937usgs.rept....5H }}</ref>

==Chemistry==
[[File:GM Guelma Hammam Challala01.jpg|thumb|right|[[Hammam Maskhoutine]] in [[Algeria]], an example of a bicarbonate hot spring]]
Because heated water can hold more [[solvation|dissolved]] [[solid]]s than cold water, the water that issues from hot springs often has a very high [[mineral]] content, containing everything from [[calcium]] to [[lithium]] and even [[radium]]. The overall chemistry of hot springs varies from ''alkaline chloride'' to ''acid sulfate'' to ''bicarbonate'' to ''iron-rich'', each of which defines an end member of a range of possible hot spring chemistries.<ref>{{cite journal |last1=Drake |first1=Bryan D. |last2=Campbell |first2=Kathleen A. |last3=Rowland |first3=Julie V. |last4=Guido |first4=Diego M. |last5=Browne |first5=Patrick R.L. |last6=Rae |first6=Andrew |title=Evolution of a dynamic paleo-hydrothermal system at Mangatete, Taupo Volcanic Zone, New Zealand |journal=Journal of Volcanology and Geothermal Research |date=August 2014 |volume=282 |pages=19–35 |doi=10.1016/j.jvolgeores.2014.06.010|bibcode=2014JVGR..282...19D |hdl=11336/31453 |hdl-access=free }}</ref>{{sfn|Des Marais|Walter|2019}}

Alkaline chloride hot springs are fed by hydrothermal fluids that form when groundwater containing dissolved [[chloride]] salts reacts with silicate rocks at high temperature. These springs have nearly neutral [[pH]] but are saturated with [[silica]] ({{chem2|SiO2}}). The solubility of silica depends strongly upon temperature, so upon cooling, the silica is deposited as [[geyserite]], a form of [[opal]] (opal-A: {{chem2|SiO2·nH2O}}).<ref>{{cite journal |last1=White |first1=Donald E. |last2=Brannock |first2=W.W. |last3=Murata |first3=K.J. |title=Silica in hot-spring waters |journal=Geochimica et Cosmochimica Acta |date=August 1956 |volume=10 |issue=1–2 |pages=27–59 |doi=10.1016/0016-7037(56)90010-2|bibcode=1956GeCoA..10...27W }}</ref> This process is slow enough that geyserite is not all deposited immediately around the vent, but tends to build up a low, broad platform for some distance around the spring opening.{{sfn|Drake|Campbell|Rowland|Guido|2014}}{{sfn|Des Marais|Walter|2019}}<ref>{{cite journal |last1=White |first1=D.E. |last2=Thompson |first2=G.A. |last3=Sandberg |first3=C.H. |title=Rocks, structure, and geologic history of Steamboat Springs thermal area, Washoe County, Nevada |journal=U.S. Geological Survey Professional Paper |series=Professional Paper |date=1964 |volume=458-B |doi=10.3133/pp458B|doi-access=free |bibcode=1964usgs.rept...19W }}</ref>

Acid sulfate hot springs are fed by hydrothermal fluids rich in [[hydrogen sulfide]] ({{chem2|H2S}}), which is oxidized to form [[sulfuric acid]], {{chem2|H2SO4}}.{{sfn|Drake|Campbell|Rowland|Guido|2014}} The pH of the fluids is thereby lowered to values as low as 0.8.<ref>{{cite journal |last1=Cox |first1=Alysia |last2=Shock |first2=Everett L. |last3=Havig |first3=Jeff R. |title=The transition to microbial photosynthesis in hot spring ecosystems |journal=Chemical Geology |date=January 2011 |volume=280 |issue=3–4 |pages=344–351 |doi=10.1016/j.chemgeo.2010.11.022|bibcode=2011ChGeo.280..344C }}</ref> The acid reacts with rock to alter it to [[clay mineral]]s, [[oxide minerals]], and a residue of silica.{{sfn|Des Marais|Walter|2019}}

Bicarbonate hot springs are fed by hydrothermal fluids that form when [[carbon dioxide]] ({{chem2|CO2}}) and groundwater react with [[carbonate rocks]].{{sfn|Drake|Campbell|Rowland|Guido|2014}} When the fluids reach the surface, {{chem2|CO2}} is rapidly lost and carbonate minerals precipitate as [[travertine]], so that bicarbonate hot springs tend to form high-relief structures around their openings.{{sfn|Des Marais|Walter|2019}}

Iron-rich springs are characterized by the presence of microbial communities that produce clumps of oxidized iron from iron in the hydrothermal fluids feeding the spring.<ref>{{cite journal |last1=Parenteau |first1=M. N. |last2=Cady |first2=S. L. |title=Microbial biosignatures in iron-mineralized phototrophic mats at Chocolate Pots Hot Springs, Yellowstone National Park, United States |journal=PALAIOS |date=2010-02-01 |volume=25 |issue=2 |pages=97–111 |doi=10.2110/palo.2008.p08-133r|bibcode=2010Palai..25...97P |s2cid=128592574 }}</ref>{{sfn|Des Marais|Walter|2019}}

Some hot springs produce fluids that are intermediate in chemistry between these extremes. For example, mixed acid-sulfate-chloride hot springs are intermediate between acid sulfate and alkaline chloride springs and may form by mixing of acid sulfate and alkaline chloride fluids. They deposit geyserite, but in smaller quantities than alkaline chloride springs.{{sfn|Drake|Campbell|Rowland|Guido|2014}}

==Flow rates==
[[File:Islande source Deildartunguhver.jpg|thumb|upright|[[Deildartunguhver]], [[Iceland]]: the highest flow hot spring in [[Europe]]]]

Hot springs range in flow rate from the tiniest "seeps" to veritable rivers of hot water. Sometimes there is enough pressure that the water shoots upward in a [[geyser]], or [[fountain]].

===High-flow hot springs===
There are many claims in the literature about the flow rates of hot springs. There are many more high flow non-thermal springs than geothermal springs. Springs with high flow rates include:
* The [[Dalhousie Springs]] complex in Australia had a peak total flow of more than 23,000&nbsp;liters/second in 1915, giving the average spring in the complex an output of more than 325&nbsp;liters/second. This has been reduced now to a peak total flow of 17,370&nbsp;liters/second so the average spring has a peak output of about 250&nbsp;liters/second.<ref>{{cite journal|title=Desert Springs of Great Australian Arterial Basin |author=W. F. Ponder |author-link=Winston Ponder |journal=Conference Proceedings. Spring-fed Wetlands: Important Scientific and Cultural Resources of the Intermountain Region |year=2002 |url=http://www.wetlands.dri.edu |access-date=2013-04-06 |archive-url=https://web.archive.org/web/20081006151305/http://www.wetlands.dri.edu/ |archive-date=2008-10-06 }}</ref>
* [[File:natural iron hot spring.jpg|upright|thumb|"Blood Pond" hot spring in [[Beppu]], Japan ]]The 2,850 hot springs of [[Beppu]] in Japan are the highest flow hot spring complex in Japan. Together the Beppu hot springs produce about 1,592&nbsp;liters/second, or corresponding to an average hot spring flow of 0.56&nbsp;liters/second.
* The 303 hot springs of [[Kokonoe, Ōita|Kokonoe]] in Japan produce 1,028&nbsp;liters/second, which gives the average hot spring a flow of 3.39&nbsp;liters/second.
* [[Ōita Prefecture]] has 4,762 hot springs, with a total flow of 4,437&nbsp;liters/second, so the average hot spring flow is 0.93&nbsp;liters/second.
* The highest flow rate hot spring in Japan is the [[Tamagawa Hot Spring]] in [[Akita Prefecture]], which has a flow rate of 150 liters/second. The Tamagawa Hot Spring feeds a {{convert|3|m|ft|abbr=on}} wide stream with a temperature of {{convert|98|°C|°F}}.
* The most famous hot springs of [[Brazil]]'s [[Caldas Novas]] ("New Hot Springs" in [[Portuguese language|Portuguese]]) are tapped by 86 wells, from which 333&nbsp;liters/second are pumped for 14 hours per day. This corresponds to a peak average flow rate of 3.89&nbsp;liters/second per well.{{Citation needed|date=September 2009}}
* In [[Florida]], there are 33 recognized "magnitude one [[Spring (hydrosphere)|springs]]" (having a flow in excess of {{Convert|2800|L/s|abbr=on}}). [[Silver Springs, Florida]] has a flow of more than {{Convert|21000|L/s|abbr=on}}.
* The [[Excelsior Geyser]] Crater in [[Yellowstone National Park]] [[yield (hydrology)|yields]] about {{Convert|4000|U.S.gal/min|abbr=on}}.
* Evans Plunge in [[Hot Springs, South Dakota]] has a flow rate of {{Convert|5000|U.S.gal/min|abbr=on}} of {{Convert|87|F}} spring water. The Plunge, built in 1890, is the world's largest natural warm water indoor swimming pool.
* The hot spring of [[Saturnia]], Italy with around 500 liters a second<ref>[http://www.termedisaturnia.it/en/thermal-spring/hot-springs Terme di Saturnia] {{Webarchive|url=https://web.archive.org/web/20130417062507/http://www.termedisaturnia.it/en/thermal-spring/hot-springs |date=2013-04-17 }}, website</ref>
* [[Lava Hot Springs, Idaho|Lava Hot Springs]] in [[Idaho]] has a flow of 130&nbsp;liters/second.
* [[Glenwood Springs, Colorado|Glenwood Springs]] in [[Colorado]] has a flow of 143&nbsp;liters/second.
* [[Elizabeth Springs]] in western [[Queensland]], Australia might have had a flow of 158&nbsp;liters/second in the late 19th century, but now has a flow of about 5&nbsp;liters/second.
* [[Deildartunguhver]] in [[Iceland]] has a flow of 180&nbsp;liters/second.
* There are at least three hot springs in the [[Nage]] region {{convert|8|km|mi|abbr=on}} south west of [[Bajawa]] in [[Indonesia]] that collectively produce more than 453.6&nbsp;liters/second.
* There are another three large hot springs (Mengeruda, Wae Bana and Piga) {{convert|18|km|mi|abbr=on}} north east of Bajawa, [[Indonesia]] that together produce more than 450&nbsp;liters/second of hot water.

==Ecosystems==
{{See also|Thermophile}}
[[File:Algal mats on hot pool, Orakei Korako 1.jpg|thumb|right|[[Algal mat]]s growing in the ''Map of Africa'' hot pool, [[Orakei Korako]], New Zealand]]
Hot springs often host communities of microorganisms adapted to life in hot, mineral-laden water. These include [[thermophile]]s, which are a type of [[extremophile]] that thrives at high temperatures, between {{convert|45|and|80|°C|°F|abbr=on}}.<ref>{{cite book |title=Brock Biology of Microorganisms | year=2006|vauthors=Madigan MT, Martino JM | edition=11th |page=136 |publisher=Pearson |isbn=978-0-13-196893-6}}</ref> Further from the vent, where the water has had time to cool and precipitate part of its mineral load, conditions favor organisms adapted to less extreme conditions. This produces a succession of microbial communities as one moves away from the vent, which in some respects resembles the successive stages in the evolution of early life.{{sfn|Farmer|2000}}

For example, in a bicarbonate hot spring, the community of organisms immediately around the vent is dominated by filamentous thermophilic [[bacteria]], such as ''[[Aquifex]]'' and other [[Aquificales]], that oxidize sulfide and hydrogen to obtain energy for their life processes. Further from the vent, where water temperatures have dropped below {{convert|60|°C||}}, the surface is covered with microbial mats {{convert|1|cm||}} thick that are dominated by [[cyanobacteria]], such as ''[[Spirulina (genus)|Spirulina]]'', ''[[Oscillatoria]]'', and ''[[Synechococcus]]'',<ref>{{cite journal |last1=Pentecost |first1=Allan |title=Cyanobacteria associated with hot spring travertines |journal=Canadian Journal of Earth Sciences |date=2003-11-01 |volume=40 |issue=11 |pages=1447–1457 |doi=10.1139/e03-075|bibcode=2003CaJES..40.1447P }}</ref> and [[green sulfur bacteria]] such as ''[[Chloroflexus]]''. These organisms are all capable of [[photosynthesis]], though green sulfur bacteria produce [[sulfur]] rather than [[oxygen]] during photosynthesis. Still further from the vent, where temperatures drop below {{convert|45|°C||}}, conditions are favorable for a complex community of microorganisms that includes ''Spirulina'', ''[[Calothrix]]'', [[diatoms]] and other single-celled [[eukaryotes]], and grazing insects and protozoans. As temperatures drop close to those of the surroundings, higher plants appear.{{sfn|Farmer|2000}}

Alkali chloride hot springs show a similar succession of communities of organisms, with various thermophilic bacteria and [[archaea]] in the hottest parts of the vent. Acid sulfate hot springs show a somewhat different succession of microorganisms, dominated by acid-tolerant algae (such as members of [[Cyanidiophyceae]]), [[fungi]], and diatoms.{{sfn|Drake|Campbell|Rowland|Guido|2014}} Iron-rich hot springs contain communities of photosynthetic organisms that oxidize reduced ([[ferrous]]) iron to oxidized ([[ferric]]) iron.{{sfn|Parenteau |Cady|2010}}

Hot springs are a dependable source of water that provides a rich chemical environment. This includes [[Oxidation|reduced]] chemical species that microorganisms can oxidize as a source of energy.

== Significance to abiogenesis ==
{{Main|Abiogenesis#Hot springs}}

=== Hot spring hypothesis ===
In contrast with "[[black smokers]]" (hydrothermal vents on the ocean floor), hot springs similar to terrestrial hydrothermal fields at Kamchatka produce fluids having suitable pH and temperature for early cells and biochemical reactions. Dissolved organic compounds were found in hot springs at Kamchatka .<ref>{{Cite journal |last=Kompanichenko |first=Vladimir N. |date=May 16, 2019 |title=Exploring the Kamchatka Geothermal Region in the Context of Life's Beginning |journal=Life |language=en |volume=9 |issue=2 |page=41 |doi=10.3390/life9020041 |pmid=31100955 |pmc=6616967 |bibcode=2019Life....9...41K |issn=2075-1729|doi-access=free }}</ref><ref name=":1" /> Metal sulfides and silica minerals in these environments would act as photocatalysts.<ref name=":1">{{Cite journal |last1=Mulkidjanian |first1=Armen Y. |last2=Bychkov |first2=Andrew Yu. |last3=Dibrova |first3=Daria V. |last4=Galperin |first4=Michael Y. |last5=Koonin |first5=Eugene V. |date=2012-04-03 |title=Origin of first cells at terrestrial, anoxic geothermal fields |journal=Proceedings of the National Academy of Sciences |volume=109 |issue=14 |pages=E821-30 |doi=10.1073/pnas.1117774109 |pmc=3325685 |pmid=22331915|doi-access=free }}</ref> They experience cycles of wetting and drying which promote the formation of biopolymers which are then encapsulated in vesicles after rehydration.<ref>{{Cite journal |last1=Damer |first1=Bruce |last2=Deamer |first2=David |date=March 15, 2015 |title=Coupled Phases and Combinatorial Selection in Fluctuating Hydrothermal Pools: A Scenario to Guide Experimental Approaches to the Origin of Cellular Life |journal=Life |volume=5 |issue=1 |pages=872–887 |doi=10.3390/life5010872 |pmid=25780958 |pmc=4390883 |bibcode=2015Life....5..872D |doi-access=free }}</ref> Solar UV exposure to the environment promotes synthesis to monomeric biomolecules.<ref>{{Cite journal |last1=Patel |first1=Bhavesh H. |last2=Percivalle |first2=Claudia |last3=Ritson |first3=Dougal J. |last4=Duffy |first4=Colm D. |last5=Sutherland |first5=John D. |date=March 16, 2015 |title=Common origins of RNA, protein and lipid precursors in a cyanosulfidic protometabolism |journal=Nature Chemistry |language=en |volume=7 |issue=4 |pages=301–307 |doi=10.1038/nchem.2202 |pmid=25803468 |pmc=4568310 |bibcode=2015NatCh...7..301P |issn=1755-4349}}</ref> The ionic composition and concentration of hot springs (K, B, Zn, P, O, S, C, Mn, N, and H) are identical to the cytoplasm of modern cells and possibly to those of the [[Last universal common ancestor|LUCA]] or early cellular life according to phylogenomic analysis.<ref>{{Cite journal |last1=Van Kranendonk |first1=Martin J. |last2=Baumgartner |first2=Raphael |last3=Djokic |first3=Tara |last4=Ota |first4=Tsutomu |last5=Steller |first5=Luke |last6=Garbe |first6=Ulf |last7=Nakamura |first7=Eizo |date=2021-01-01 |title=Elements for the Origin of Life on Land: A Deep-Time Perspective from the Pilbara Craton of Western Australia |url=https://www.liebertpub.com/doi/abs/10.1089/ast.2019.2107 |journal=Astrobiology |volume=21 |issue=1 |pages=39–59 |doi=10.1089/ast.2019.2107 |pmid=33404294 |bibcode=2021AsBio..21...39V |s2cid=230783184|url-access=subscription }}</ref><ref name=":1" /> For these reasons, it has been hypothesized that hot springs may be the place of origin of life on Earth.{{sfn|Farmer|2000}}{{sfn|Des Marais|Walter|2019}} The evolutionary implications of the hypothesis imply a direct evolutionary pathway to land plants. Where continuous exposure to sunlight leads to the development of photosynthetic properties and later colonize on land and life at hydrothermal vents is suggested to be a later adaptation.<ref name=":2">{{Cite journal |last1=Damer |first1=Bruce |last2=Deamer |first2=David |date=2020-04-01 |title=The Hot Spring Hypothesis for an Origin of Life |journal=Astrobiology |volume=20 |issue=4 |pages=429–452 |doi=10.1089/ast.2019.2045 |issn=1531-1074 |pmc=7133448 |pmid=31841362|bibcode=2020AsBio..20..429D }}</ref>

Recent experimental studies at hot springs support this hypothesis. They show that fatty acids self-assemble into membranous structures and encapsulate synthesized biomolecules during exposure to UV light and multiple wet-dry cycles at slightly alkaline or acidic hot springs, which would not happen at saltwater conditions as the high concentrations of ionic solutes there would inhibit the formation of membranous structures.<ref name=":2" /><ref>{{Cite journal |last=Deamer |first=David |date= February 10, 2021 |title= Where Did Life Begin? Testing Ideas in Prebiotic Analogue Conditions |journal=Life |volume=11 |issue=2 |page=134 |doi=10.3390/life11020134 |pmid= 33578711 |pmc= 7916457 |bibcode=2021Life...11..134D |issn=2075-1729|doi-access=free }}</ref><ref>{{Cite journal |last1=Milshteyn |first1=Daniel |last2=Damer |first2=Bruce |last3=Havig |first3= Jeff |last4= Deamer |first4=David |date=May 10, 2018 |title=Amphiphilic Compounds Assemble into Membranous Vesicles in Hydrothermal Hot Spring Water but Not in Seawater |journal= Life |volume=8 |issue=2 |page=11 |doi=10.3390/life8020011 |pmid=29748464 |pmc=6027054 |bibcode=2018Life....8...11M |doi-access=free }}</ref> [[David W. Deamer|David Deamer]] and Bruce Damer note that these hypothesized prebiotic environments resemble [[Charles Darwin]]'s imagined "warm little pond".<ref name=":2"/> If life did not emerge at deep sea hydrothermal vents, rather at terrestrial pools, extraterrestrial quinones transported to the environment would generate redox reactions conducive to proton gradients. Without continuous wet-dry cycling to maintain stability of primitive proteins for membrane transport and other biological macromolecules, they would go through hydrolysis in an aquatic environment.<ref name=":2" /> Scientists discovered a 3.48 billion year old geyserite that seemingly preserved fossilized microbial life, stromatolites, and biosignatures.<ref>{{Cite journal |last1=Djokic |first1=Tara |last2=Van Kranendonk |first2=Martin J. |last3=Campbell |first3=Kathleen A. |last4=Walter |first4=Malcolm R. |last5=Ward |first5=Colin R. |date=2017-05-09 |title=Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits |journal=Nature Communications |language=en |volume=8 |issue=1 |article-number=15263 |doi=10.1038/ncomms15263 |pmid=28486437 |pmc=5436104 |bibcode=2017NatCo...815263D |issn=2041-1723}}</ref> Researchers propose pyrophosphite to have been used by early cellular life for energy storage and it might have been a precursor to pyrophosphate. Phosphites, which are present at hot springs, would have bonded together into pyrophosphite within hot springs through wet-dry cycling.<ref name=":3">{{Cite web |last=Marshall |first=Michael |date=April 2, 2013 |title= Meteorites could have been source of life's batteries |url=https://www.newscientist.com/article/mg21829114-800-meteorites-could-have-been-source-of-lifes-batteries/ |access-date=2022-11-01 |website=New Scientist |language=en-US}}</ref> Like alkaline hydrothermal vents, the Hakuba Happo hot spring goes through serpentinization, suggesting methanogenic microbial life possibly originated in similar habitats.<ref>{{Cite journal |last1=Suda |first1=Konomi |last2=Ueno |first2=Yuichiro |last3=Yoshizaki |first3=Motoko |last4=Nakamura |first4=Hitomi |last5=Kurokawa |first5=Ken |last6=Nishiyama |first6=Eri |last7=Yoshino |first7=Koji |last8=Hongoh |first8=Yuichi |last9=Kawachi |first9=Kenichi |last10=Omori |first10=Soichi |last11=Yamada |first11=Keita |last12=Yoshida |first12=Naohiro |last13= Maruyama |first13= Shigenori |date=2014-01-15 |title=Origin of methane in serpentinite-hosted hydrothermal systems: The CH4–H2–H2O hydrogen isotope systematics of the Hakuba Happo hot spring |url=https://www.sciencedirect.com/science/article/pii/S0012821X13006286 |journal=Earth and Planetary Science Letters |language=en |volume=386 |pages=112–125 |doi=10.1016/j.epsl.2013.11.001 |bibcode=2014E&PSL.386..112S |issn=0012-821X|url-access=subscription }}</ref>

=== Limitations ===
A problem with the hot spring hypothesis for an origin of life is that phosphate has low solubility in water.<ref name=":4">{{Cite journal |last1=Longo |first1=Alex |last2=Damer |first2=Bruce |date=2020-04-27 |title=Factoring Origin of Life Hypotheses into the Search for Life in the Solar System and Beyond |journal=Life |volume=10 |issue=5 |page=52 |doi=10.3390/life10050052 |issn=2075-1729 |pmc=7281141 |pmid=32349245|bibcode=2020Life...10...52L |doi-access=free }}</ref> Pyrophosphite could have been present within protocells, however all modern life forms use pyrophosphate for energy storage. Kee suggests that pyrophosphate could have been utilized after the emergence of enzymes.<ref name=":3" /> Dehydrated conditions would favor phosphorylation of organic compounds and condensation of phosphate to polyphosphate.<ref>{{Cite journal |last1=Kitadai |first1=Norio |last2=Maruyama |first2=Shigenori |date=2018-07-01 |title=Origins of building blocks of life: A review |journal=Geoscience Frontiers |language=en |volume=9 |issue=4 |pages=1117–1153 |doi=10.1016/j.gsf.2017.07.007 |bibcode=2018GeoFr...9.1117K |s2cid=102659869 |issn=1674-9871|doi-access=free }}</ref> Another problem is that solar ultraviolet radiation and frequent impacts would have inhibited habitability of early cellular life at hot springs,<ref name=":4" /> although biological macromolecules might have undergone selection during exposure to solar ultraviolet radiation<ref name=":2" /> and would have been catalyzed by photocatalytic silica minerals and metal sulfides.<ref name=":1" /> Carbonaceous meteors during the Late Heavy Bombardment would not have caused cratering on Earth as they would produce fragments upon atmospheric entry. The meteors are estimated to have been 40 to 80 meters in diameter however larger impactors would produce larger craters.<ref>{{Cite journal |last1=Pearce |first1=Ben K. D. |last2=Pudritz |first2=Ralph E. |last3=Semenov |first3=Dmitry A. |last4=Henning |first4=Thomas K. |date=2017-10-24 |title=Origin of the RNA world: The fate of nucleobases in warm little ponds |journal=Proceedings of the National Academy of Sciences |language=en |volume=114 |issue=43 |pages=11327–11332 |doi=10.1073/pnas.1710339114 |issn=0027-8424 |pmc=5664528 |pmid=28973920|arxiv=1710.00434 |bibcode=2017PNAS..11411327P |doi-access=free }}</ref> Metabolic pathways have not yet been demonstrated at these environments,<ref name=":4" /> but the development of proton gradients might have been generated by redox reactions coupled to meteoric quinones or protocell growth.<ref>{{Cite journal |last1=Chen |first1=Irene A. |last2=Szostak |first2=Jack W. |date=2004-05-25 |title=Membrane growth can generate a transmembrane pH gradient in fatty acid vesicles |journal=Proceedings of the National Academy of Sciences |language=en |volume=101 |issue=21 |pages=7965–7970 |doi=10.1073/pnas.0308045101 |issn=0027-8424 |pmc=419540 |pmid=15148394|bibcode=2004PNAS..101.7965C |doi-access=free }}</ref><ref name=":2" /><ref>{{Cite journal |last1=Milshteyn |first1=Daniel |last2=Cooper |first2=George |last3=Deamer |first3=David |date=2019-08-28 |title=Chemiosmotic energy for primitive cellular life: Proton gradients are generated across lipid membranes by redox reactions coupled to meteoritic quinones |journal=Scientific Reports |language=en |volume=9 |issue=1 |page=12447 |doi=10.1038/s41598-019-48328-5 |pmid=31462644 |pmc=6713726 |bibcode=2019NatSR...912447M |issn=2045-2322}}</ref> Metabolic reactions in the Wood-Ljungdahl pathway and reverse Krebs cycle have been produced in acidic conditions and thermophilic temperatures in the presence of metals which is consistent with observations of RNA mostly stable at acidic pH.<ref>{{Cite journal |last1=Varma |first1=Sreejith J. |last2=Muchowska |first2=Kamila B. |last3=Chatelain |first3=Paul |last4=Moran |first4=Joseph |date=April 23, 2018 |title=Native iron reduces CO2 to intermediates and end-products of the acetyl-CoA pathway |journal=Nature Ecology & Evolution |language=en |volume=2 |issue=6 |pages=1019–1024 |doi=10.1038/s41559-018-0542-2 |pmid=29686234 |pmc=5969571 |bibcode=2018NatEE...2.1019V |issn=2397-334X}}</ref><ref>{{Cite journal |last1=Muchowska |first1=Kamila B. |last2=Varma |first2=Sreejith J. |last3=Chevallot-Beroux |first3=Elodie |last4=Lethuillier-Karl |first4=Lucas |last5=Li |first5=Guang |last6=Moran |first6=Joseph |date=October 2, 2017 |title=Metals promote sequences of the reverse Krebs cycle |journal=Nature Ecology & Evolution |language=en |volume=1 |issue=11 |pages=1716–1721 |doi=10.1038/s41559-017-0311-7 |pmid=28970480 |pmc=5659384 |bibcode=2017NatEE...1.1716M |issn=2397-334X}}</ref>

==Human uses==
[[File:Jigokudani hotspring in Nagano Japan 001.jpg|thumb|right|[[Japanese macaque|Macaque]]s enjoying an open air hot spring or "[[onsen]]" in [[Nagano, Nagano|Nagano]]]]
[[File:Tsurunoyu onsen rotenburo2.JPG|thumb|Winter bathing at Tsuru-no-yu roten-buro in Nyūtō, Akita]]
[[File:Sai ngam hot spring pai.jpg|thumb|Sai Ngam hot springs in Mae Hong Son province, Thailand]]
===Bathing===
====History====
Hot springs have been enjoyed by humans for thousands of years.<ref>{{cite journal |last1=van Tubergen |first1=A |title=A brief history of spa therapy |journal=Annals of the Rheumatic Diseases |date=1 March 2002 |volume=61 |issue=3 |pages=273–275 |doi=10.1136/ard.61.3.273|pmid=11830439 |pmc=1754027 }}</ref> Even [[macaques]] are known to have extended their northern range into [[Japan]] by making use of hot springs to protect themselves from cold stress.<ref>{{cite journal |last1=Takeshita |first1=Rafaela S. C. |last2=Bercovitch |first2=Fred B. |last3=Kinoshita |first3=Kodzue |last4=Huffman |first4=Michael A. |title=Beneficial effect of hot spring bathing on stress levels in Japanese macaques |journal=Primates |date=May 2018 |volume=59 |issue=3 |pages=215–225 |doi=10.1007/s10329-018-0655-x|pmid=29616368 |s2cid=4568998 }}</ref> Hot spring baths (''[[onsen]]'') have been in use in Japan for at least two thousand years, traditionally for cleanliness and relaxation, but increasingly for their therapeutic value.<ref>{{cite journal |last1=Serbulea |first1=Mihaela |last2=Payyappallimana |first2=Unnikrishnan |title=Onsen (hot springs) in Japan—Transforming terrain into healing landscapes |journal=Health & Place |date=November 2012 |volume=18 |issue=6 |pages=1366–1373 |doi=10.1016/j.healthplace.2012.06.020|pmid=22878276 }}</ref> In the [[Homeric Age]] of Greece (ca. 1000 BCE), baths were primarily for hygiene, but by the time of [[Hippocrates]] (ca. 460 BCE), hot springs were credited with healing power. The popularity of hot springs has fluctuated over the centuries since, but they are now popular around the world.{{sfn|van Tubergen|2002}} In 2023 the Global Wellness Institute, a [[Wellness tourism|wellness industry]] study, estimated the global earnings of the 31,200 hot springs establishments to be over $62 billion USD.<ref>Andrea Sachs. (14 March 2025). "Soak in these 9 hot springs destinations from the Yukon to Mexico." [https://wapo.st/4ib2Qhy Washington Post website] Retrieved 16 March 2025.</ref>

====Therapeutic uses====
Because of both the [[folklore]] and the claimed [[medical]] value attributed to some hot springs, they are often popular [[tourist]] destinations, and locations for [[physical therapy|rehabilitation]] [[clinic]]s for those with [[disabilities]]. However, the scientific basis for therapeutic bathing in hot springs is uncertain. Hot bath therapy for [[lead poisoning]] was common and reportedly highly successful in the 18th and 19th centuries, and may have been due to [[diuresis]] (increased production of urine) from sitting in hot water, which increased excretion of lead; better food and isolation from lead sources; and increased intake of calcium and iron. Significant improvement in patients with [[rheumatoid arthritis]] and [[ankylosing spondylitis]] have been reported in studies of spa therapy, but these studies have methodological problems, such as the obvious impracticality of [[placebo-controlled studies]] (in which a patient does not know if they are receiving the therapy). As a result, the therapeutic effectiveness of hot spring therapy remains uncertain.{{sfn|van Tubergen|2002}}

====Precautions====
Hot springs in volcanic areas are often at or near the [[boiling point]]. People have been seriously scalded and even killed by accidentally or intentionally entering these springs.<ref>{{cite web |title=Safety |url=https://www.nps.gov/yell/planyourvisit/safety.htm |website=Yellowstone National Park |publisher=National Park Service |access-date=24 June 2021 |date=8 June 2021}}</ref><ref>{{cite web |last1=Almasy |first1=Steve |title=Man severely burned after falling into Yellowstone hot spring |url=https://www.cnn.com/2017/06/14/us/yellowstone-hot-spring-burns-north-carolina-man/index.html |website=CNN |access-date=24 June 2021 |date=15 June 2017}}</ref><ref>{{cite web |last1=Andrews |first1=Robin |title=This Is What Happens When You Fall Into One Of Yellowstone's Hot Springs |url=https://www.forbes.com/sites/robinandrews/2016/12/30/this-is-what-happens-when-you-fall-into-one-of-yellowstones-hot-springs/ |website=Forbes |access-date=24 June 2021 |date=30 December 2016}}</ref>

Some hot springs microbiota are infectious to humans:
* ''[[Naegleria fowleri]]'', an [[excavata|excavate]] [[amoeba]], lives in warm unsalted waters worldwide and causes a fatal [[meningitis]] should the organisms enter the nose.<ref>{{EMedicine|article|972044|Naegleria}}</ref><ref>{{cite journal |author1=Shinji Izumiyama |author2=Kenji Yagita |author3=Reiko Furushima-Shimogawara |author4=Tokiko Asakura |author5=Tatsuya Karasudani |author6=Takuro Endo |title=Occurrence and Distribution of ''Naegleria'' Species in Thermal Waters in Japan |journal=J Eukaryot Microbiol |volume=50 |pages=514–5 |date=July 2003 |pmid=14736147 |doi=10.1111/j.1550-7408.2003.tb00614.x |s2cid=45052636 }}</ref><ref>{{cite journal |author1=Yasuo Sugita |author2=Teruhiko Fujii |author3=Itsurou Hayashi |author4=Takachika Aoki |author5=Toshirou Yokoyama |author6=Minoru Morimatsu |author7=Toshihide Fukuma |author8=Yoshiaki Takamiya |title=Primary amebic meningoencephalitis due to ''Naegleria fowleri'': An autopsy case in Japan |journal=Pathology International |volume=49 |issue=5 |pages=468–70 |date=May 1999 |pmid=10417693 |doi=10.1046/j.1440-1827.1999.00893.x |s2cid=21576553 }}</ref>
* ''[[Acanthamoeba]]'' also can spread through hot springs, according to the US [[Centers for Disease Control]] - The organisms enter through the eyes or via an open wound.<ref>[https://www.cdc.gov/Ncidod/dpd/parasites/acanthomoeba/factsht_acanthamoeba.htm CDC description of acanthamoeba]</ref>
* ''[[Legionella]]'' [[bacteria]] have been spread through hot springs.<ref>{{cite journal |vauthors=Miyamoto H, Jitsurong S, Shiota R, Maruta K, Yoshida S, Yabuuchi E |title=Molecular determination of infection source of a sporadic ''Legionella pneumonia'' case associated with a hot spring bath |journal=Microbiol. Immunol. |volume=41 |issue=3 |pages=197–202 |year=1997 |pmid=9130230 |doi=10.1111/j.1348-0421.1997.tb01190.x|s2cid=25016946 |doi-access=free }}</ref><ref>{{cite journal |author1=Eiko Yabauuchi |author2=Kunio Agata |title=An outbreak of legionellosis in a new facility of hot spring Bath in Hiuga City |journal=Kansenshogaku Zasshi |volume=78 |issue=2 |pages=90–8 |year=2004 |issn=0387-5911 |pmid=15103899|doi=10.11150/kansenshogakuzasshi1970.78.90 |doi-access=free }}</ref>
*''[[Neisseria gonorrhoeae]]'' was reported to have very likely been acquired from bathing in a hot spring according to one [[case study]], with the near-body temperature, slightly acidic, [[Isotonic fluid|isotonic]], organic matter-containing waters thought to facilitate the survival of the pathogen.<ref>{{Cite journal|last1=Goodyear-Smith|first1=Felicity|last2=Schabetsberger|first2=Robert|date=2021-09-17|title=Gonococcus infection probably acquired from bathing in a natural thermal pool: a case report|journal=Journal of Medical Case Reports|volume=15|issue=1|page=458|doi=10.1186/s13256-021-03043-6|issn=1752-1947|pmc=8445652|pmid=34530901 |doi-access=free }}</ref>

====Etiquette====
The customs and practices observed differ depending on the hot spring. It is common practice that bathers should wash before entering the water so as not to contaminate the water (with/without soap).<ref>{{Cite book
 |title=Ryokan
 |first=Gabriele |last=Fahr-Becker
 |year=2001
 |isbn=978-3-8290-4829-3
 |page=24
|publisher=Könemann }}</ref> In many countries, like Japan, it is required to enter the hot spring with no clothes on, including swimwear. Often there are different facilities or times for men and women, but mixed ''onsen'' do exist.<ref>{{cite web |last1=Cheung |first1=Jeanne |title=A Guide to Japan's Onsen Etiquette for First Timers (Hint: You're Gonna be in the Buff) |url=https://traveler.marriott.com/culture-and-style/onsen-japan-etiquette-for-first-timers/ |website=Marriot Bonvoy Traveler |date=16 February 2018 |publisher=Marriot Internal Inc. |access-date=2 July 2021}}</ref> In some countries, if it is a public hot spring, swimwear is required.<ref>{{cite web |title=Spa Etiquette & Information |url=https://www.onespa.com/spa-information/ |website=One Spa |access-date=2 July 2021}}</ref><ref>{{cite web |title=Nudity Spa Guide |url=https://www.spafinder.com/nudity-spa-guide/ |website=Spa Finder |date=19 July 2016 |publisher=Blackhawk Network, Inc. |access-date=2 July 2021}}</ref>


===Cooking and drinking===
{{Anchor|Cooking}}Hot springs are used for {{Ill|geothermal cooking|ja|温泉地熱料理}}, such as boiling eggs and vegetables at [[Hammam Maskhoutine]] (Algeria),<ref name="Powell1926">{{Cite book |title=In Barbary; Tunisia, Algeria, Morocco, and the Sahara |last=Powell |first=E. Alexander |publisher=Century Co. |year=1926 |location=New York, US |pages=260 |language=en |author-link=E. Alexander Powell |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015027021479&seq=286&q1=%22boiling+their+eggs%22 |hdl=2027/mdp.39015027021479 |oclc=1079292280}}</ref> and in Japan.<ref name="BBC2020">{{Cite web |title=The Japanese village that cooks in a hot spring |url=https://www.bbc.com/travel/article/20200824-the-japanese-village-that-cooks-in-a-hot-spring |last=Nakamoto |first=Michiyo |date=2020-08-26 |url-status=live |archive-url=https://web.archive.org/web/20250426153818/https://www.bbc.com/travel/article/20200824-the-japanese-village-that-cooks-in-a-hot-spring |archive-date=2025-04-26 |access-date=2025-10-17 |website=[[BBC]]}}</ref> Immersing eggs into hot springs of around {{convert|67|C|F}} produces {{transliteration|ja|[[onsen tamago]]}} ({{literal translation|hot spring egg}}), which have unique [[food texture|texture]] unlike ordinary [[boiled egg]]s, with a soft-set yolk<ref name="JapanTimes2014">{{Cite web |title=The raw appeal of eggs |url=https://www.japantimes.co.jp/life/2014/09/16/food/raw-appeal-eggs/ |last=Itoh |first=Makiko |date=2014-09-16 |url-status=live |archive-url=https://web.archive.org/web/20140925025909/https://www.japantimes.co.jp/life/2014/09/16/food/raw-appeal-eggs/ |archive-date=2014-09-25 |access-date=2025-10-11 |website=[[Japan Times]]}}</ref> and custardy white.<ref name="WashingtonPost20240901">{{Cite web |title=How ancient healing hot springs could fuel a clean energy future |url=https://www.washingtonpost.com/climate-solutions/2024/09/01/japan-geothermal-energy-onsen/ |last=Rivero |first=Nicolás |date=2024-09-01 |url-status=live |archive-url=https://web.archive.org/web/20240904130759/https://www.washingtonpost.com/climate-solutions/2024/09/01/japan-geothermal-energy-onsen/ |archive-date=2024-09-04 |access-date=2025-10-17 |website=[[Washington Post]] |last2=Inuma |first2=Julia Mio}}</ref> {{transliteration|ja|Onsen tamago}} are served in hot spring resorts throughout Japan.<ref name="JNTO">{{Cite web |title=Guide to Hot Springs in Japan’s National Parks |url=https://www.japan.travel/national-parks/plan-your-visit/guides-and-stories/guide-hot-springs-japans-national-parks/ |last=Munro |first=Kirsty |url-status=live |archive-url=https://web.archive.org/web/20251002145523/https://www.japan.travel/national-parks/plan-your-visit/guides-and-stories/guide-hot-springs-japans-national-parks/ |archive-date=2025-10-02 |access-date=2025-10-17 |publisher=[[Japan National Tourism Organization]] |language=en}}</ref>

{{Anchor|Drinking}}Some people drink the water from hot springs as [[folk remedies]].<ref name="Powell1926"/><ref name="BBC2020"/>

==Examples==
[[File:Geothermal springs map US.png|thumb|upright=1.3|Distribution of [[Geothermal activity|geothermal]] springs in the US]]

{{Main|List of hot springs}}
There are hot springs in many places and on all continents of the world. Countries that are renowned for their hot springs include [[China]], [[Costa Rica]], [[List of spa towns in Hungary|Hungary]], [[Iceland]], [[Iran]], [[Onsen|Japan]], [[New Zealand]], [[Brazil]], [[Peru]], [[List of spa towns in Serbia|Serbia]], [[South Korea]], [[Taiwan]], [[Turkey]], and the [[United States]], but there are hot springs in many other places as well:
* Widely renowned since a chemistry professor's report in 1918 classified them as one of the world's most [[electrolytic]] mineral waters, the [[Termas de Rio Hondo|Rio Hondo Hot Springs]] in northern [[Argentina]] have become among the most visited on earth.<ref>[http://www.welcomeargentina.com/termasderiohondo Welcome Argentina: Turismo en Argentina 2009]</ref> The [[Cacheuta Spa]] is another famous hot springs in Argentina.
* The springs in Europe with the highest temperatures are located in France, in a small village named [[Chaudes-Aigues]].{{citation needed|date=April 2017}} Located at the heart of the French volcanic region [[Auvergne]], the thirty natural hot springs of Chaudes-Aigues have temperatures ranging from {{cvt|45|C}} to more than {{cvt|80|C}}. The hottest one, the "Source du Par", has a temperature of {{cvt|82|C}}. The hot waters running under the village have provided heat for the houses and for the church since the 14th Century. Chaudes-Aigues (Cantal, France) is a [[spa town]] known since the Roman Empire for the treatment of rheumatism.
* Carbonate aquifers in foreland tectonic settings can host important thermal springs although located in areas commonly not characterised by regional high heat flow values. In these cases, when thermal springs are located close or along the coastlines, the subaerial and/or submarine thermal springs constitute the outflow of marine groundwater, flowing through localised fractures and karstic rock-volumes. This is the case of springs occurring along the south-easternmost portion of the Apulia region (Southern Italy) where few sulphurous and warm waters ({{cvt|22|-|33|C}}) outflow in partially submerged caves located along the Adriatic coast, thus supplying the historical spas of Santa Cesarea Terme. These springs are known from ancient times (Aristotele in III Century BC) and the physical-chemical features of their thermal waters resulted to be partly influenced by the sea level variations.<ref>{{Cite journal|last1=Santaloia|first1=F.|last2=Zuffianò|first2=L. E.|last3=Palladino|first3=G.|last4=Limoni|first4=P. P.|last5=Liotta|first5=D.|last6=Minissale|first6=A.|last7=Brogi|first7=A.|last8=Polemio|first8=M.|date=2016-11-01|title=Coastal thermal springs in a foreland setting: The Santa Cesarea Terme system (Italy)|journal=Geothermics|volume=64|pages=344–361|doi=10.1016/j.geothermics.2016.06.013|bibcode=2016Geoth..64..344S |hdl=11586/167990 |issn=0375-6505|hdl-access=free}}</ref>
* One of the potential geothermal energy reservoirs in India is the Tattapani thermal springs of Madhya Pradesh.<ref>{{cite journal|year=1987|title=Geothermal studies at Tattapani hot spring area, Sarguja district, central India|journal=Geothermics|volume=16|issue=1|pages=61–76|doi=10.1016/0375-6505(87)90079-4|author1=Ravi Shanker|author2=J.L. Thussu|author3=J.M. Prasad|bibcode=1987Geoth..16...61S }}</ref><ref>{{cite journal|date=August 1995|title=Geochemistry of Tattapani thermal springs, Himachal Pradesh, India—field and experimental investigations|journal=Geothermics|volume=24|issue=4|pages=553–9|doi=10.1016/0375-6505(95)00005-B|author1=D. Chandrasekharam|author2=M.C. Antu}}</ref>
* The silica-rich deposits found in [[Nili Patera]], the [[Volcano|volcanic]] [[caldera]] in [[Syrtis Major Planum|Syrtis Major]], [[Mars]], are thought to be the remains of an extinct hot spring system.<ref>{{Cite journal|last1=Skok|first1=J. R.|last2=Mustard|first2=J. F.|last3=Ehlmann|first3=B. L.|last4=Milliken|first4=R. E.|last5=Murchie|first5=S. L.|date=December 2010|title=Silica deposits in the Nili Patera caldera on the Syrtis Major volcanic complex on Mars|url=http://www.nature.com/articles/ngeo990|journal=Nature Geoscience|language=en|volume=3|issue=12|pages=838–841|doi=10.1038/ngeo990|bibcode=2010NatGe...3..838S|issn=1752-0894|url-access=subscription}}</ref>

==See also==
{{Commons category|Hot springs}}
{{Wikivoyage|Hot springs}}
{{div col|colwidth=20em}}
* [[Deep Creek Hot Springs]]
* [[Hotspot (geology)]]
* [[Hydrothermal vent]]s
* [[Earliest known life forms]]
* [[List of spa towns]]
* [[List of hot springs]] (worldwide listing)
* [[List of hot springs in Japan]]
* [[List of hot springs in the United States]]
* [[Mineral spring]]
* [[Valley of Geysers]]
{{div col end}}

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

==Further reading==
* {{cite book |author=Marjorie Gersh-Young |title=Hot Springs and Hot Pools of the Southwest: Jayson Loam's Original Guide |publisher=Aqua Thermal Access |year=2011 |isbn=978-1-890880-07-1 }}
* {{cite book |author=Marjorie Gersh-Young |title=Hot Springs & Hot Pools Of The Northwest |publisher=Aqua Thermal Access |year=2008 |isbn=978-1-890880-08-8 |url-access=registration |url=https://archive.org/details/isbn_9781890880088 }}
* {{cite book |author=G. J Woodsworth |title=Hot springs of Western Canada: a complete guide |publisher=Gordon Soules |location=West Vancouver |year=1999 |isbn=978-0-919574-03-8 }}
* {{cite news |author=Clay Thompson |title=Tonopah: It's Water Under The Bush |newspaper=Arizona Republic |page=B12 |date=2003}}

==External links==
{{Wikivoyage|Hot springs}}
* [https://www.ngdc.noaa.gov/hazard/data/publications/NCEI-thermal-springs.xlsx Thermal Springs List for the United States]—1,661 hot springs
* {{cite journal |title=Geothermal Resources of the Great Artesian Basin, Australia |journal=GHC Bulletin |volume=23 |issue=2 |date=June 2002 |url=http://geoheat.oit.edu/bulletin/bull23-2/art5.pdf |access-date=2006-11-02 |archive-date=2014-08-22 |archive-url=https://web.archive.org/web/20140822165751/http://geoheat.oit.edu/bulletin/bull23-2/art5.pdf }}
* [https://web.archive.org/web/20061110183857/http://www.os.is/Apps/WebObjects/Orkustofnun.woa/swdocument/1883/04+Bahati+IGC+2003.pdf A scholarly paper with a map of over 20 geothermal areas in Uganda]
* [http://www.nzhotpools.co.nz List of 100 thermal hot springs and hot pools in New Zealand]
* [http://www.hotspringsworldwide.info/worldwide.html List of hot springs worldwide]

{{Rivers, streams and springs}}
{{Authority control}}

[[Category:Hot springs| ]]
[[Category:Bathing]]
[[Category:Springs (hydrology)]]
[[Category:Bodies of water]]
[[Category:Geothermal areas]]