Hot springs
A hot spring is a spring that is produced by the emergence of geothermally-heated groundwater from the  earth's crust.   There are hot springs all over the earth, on every continent and even under the oceans and seas.

# Definitions
There is no universally accepted definition of a hot spring. For example, one can find the phrase hot spring defined as
- any geothermal spring
- a spring with water temperatures above its surroundings
- a natural spring with water temperature above body temperature  normally between 36.5 C (97.7 F) and 37.5 C (99.5 F)
- a natural spring with warm water above body temperature
- a thermal spring with water warmer than 36.7 C (98.1 F) 
- a natural spring of water greater than 21.1 C (70 F) (synonymous with thermal spring) 
- a natural discharge of groundwater with elevated temperatures
- a type of thermal spring in which hot water is brought to the surface. The water temperature of a hot spring is usually 6.5 C (11.7 F) or more above mean air temperature.  Note that by this definition, "thermal spring" is not synonymous with the term "hot spring".
- a spring whose hot water is brought to the surface (synonymous with a thermal spring). The water temperature of the spring is usually 8.3 C (14.9 F) or more above the mean air temperature.
- a spring with water above the core human body temperature  36.7 C (98.1 F).
- a spring with water above average ambient ground temperature, a definition favored by some.
- a spring with water temperatures above 50 C (122 F)
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. The US NOAA Geophysical Data Center defines a "warm spring" as a spring with water between 20 C (68 F) and 50 C (122 F). The  symbol is commonly used on maps to denote a hot spring.

# Sources of heat
The water issuing from a hot spring is heated by geothermal heat,  i.e., heat from the Earth's interior.  In general, 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 rocks. The water from hot springs in non-volcanic areas is heated in this manner.
In volcanic zones such as Yellowstone National Park, water may be heated by coming into contact with magma (molten rock).  The high temperature gradient near magma may cause water to be heated enough that it boils or becomes superheated. If the water becomes so hot that it builds steam pressure and erupts in a jet above the surface of the Earth, it is called a geyser. If the water only reaches the surface in the form of steam, it is called a fumarole. If the water is mixed with mud and clay, it is called a mud pot.
Note that hot springs in volcanic areas are often at or near the boiling point. People have been seriously burned and even killed by accidentally or intentionally entering these springs.
Warm springs are sometimes the result of hot and cold springs mixing but may also occur outside of volcanic areas, such as Warm Springs, Georgia (frequented for its therapeutic effects by paraplegic U.S. President Franklin D. Roosevelt, who built the Little White House there).

# Flow rates
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.
A very low flow rate hot spring fed the closed resort, Fales Hot Ditch, which is north of Bridgeport, California. There is a huge subterranean lake below Tonopah, Arizona, which provides natural hot mineral waters to several hot springs. These hot springs were used by the seven or more hot spring spas that once operated in Tonopah. The ruins of two such spas are still visible in Tonopah.

## High flow hot springs
There are many claims in the literature about the flow rates of hot springs. Some of the hot springs with high flow rates and high claimed flow rates. It should be noted that there are many more very high flow nonthermal springs than geothermal springs. For example, there are 33 recognized "magnitude one springs" (having a flow in excess of 2,800 liters/second) in Florida alone. Silver Springs, Florida has a flow of more than 21,000 liters/second. Springs with high flow rates include:
- The combined flow of the 47 hot springs in Hot Springs, Arkansas is 35 liters/second.
- The combined flow of the hot springs complex in Truth or Consequences, New Mexico is estimated at 99 liters/second.
- Lava Hot Springs in Idaho has a flow of 130 liters/second.
- Glenwood Springs in Colorado has a flow of 143 liters/second.
- Elizabeth Springs in western Queensland, Australia might have had a flow of 158 liters/second in the late 1800s, but now has a flow of about 5 liters/second
- Deildartunguhver in Iceland has a flow of 180 liters/second.
- The hot springs of Brazil's Caldas Novas ("Hot River" in Portuguese) are tapped by 86 wells, from which 333 liters/second are pumped for 14 hours per day. This corresponds to a peak average flow rate of 3.89 liters/second per well.
- 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 liters/second, or corresponding to  an average hot spring flow of 0.56 liters/second.
- The 303 hot springs of Kokonoe in Japan produce 1,028 liters/second, which gives the average hot spring a flow of 3.39 liters/second.
- The Oita Prefecture has 4,762 hot springs, with a total flow of 4,437 liters/second, so the average hot spring flow is 0.93 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 3 m (9.8425197 ft) wide stream with a temperature of 98 C (208.4 F).
- There are at least three hot springs in the Nage region 8 km (4.970969536 mi) south west of Bajawa City in Indonesia that collectively produce more than 453.6 liters/second.
- There are another three large hot springs (Mengeruda, Wae Bana and Piga) 18 km (11.184681456 mi) north east of Bajawa City, Indonesia that together produce more than 450 liters/second of hot water.
- The Dalhousie Springs complex in Australia had a peak total flow of more than 23,000 liters/second in 1915, giving the average spring in the complex an output of more than 325 liters/second. This has been reduced now to a peak total fow of 17,370 liters/second so the average spring has a peak output of about 250 liters/second.

# Therapeutic uses
Because heated water can hold more dissolved solids, warm and especially hot springs also often have a very high mineral content, containing everything from simple calcium to lithium, and even radium.  Because of both the folklore and the claimed medical value some of these springs have, they are often popular tourist destinations, and locations for rehabilitation clinics for those with disabilities.

# Infections from hot springs
Unfortunately, hot springs can create ideal conditions to spread infections. For example:
- Naegleria fowleri, an amoeba, lives in warm waters and soils worldwide and can be a cause of meningitis. Several deaths have been attributed to this amoeba, which enters the brain through the nasal passages.
- Acanthamoeba also can spread through hot springs, according to the US Centers for Disease Control.
- Legionella bacteria have been spread through hot springs.
- A bather may have been infected by the herpes simplex virus in his toe from a hot spring in Japan.
- Viruses have been collected from very extreme environments, for example, a hot spring with a temperature of 87 C (188.6 F) to 93 C (199.4 F) and an incredibly acidic pH of 1.5 in Pozzuoli, Italy. These viruses were observed to infect cells in the laboratory.

# Hot springs around the world
There are hot springs on all continents and in many countries around the world. Countries that are renowned for their hot springs include Iceland, New Zealand, Chile, Canada, Taiwan,
and Japan, but there are interesting and unique hot springs in many other places as well:
- The town of Spa, Belgium is the origin of the word "spa" and features springs with water temperatures of 32 C (89.6 F). Casanova visited Spa in 1783 looking for business opportunities but was disappointed.
- Aachen, Germany has the hottest springs of Central Europe with water temperatures of 74 C (165.2 F).
- There are more than 275 hot springs registered in Chile including South America's largest hot spring source in Liquine.
- The Yangbajing hot springs field about 87 km. north of Lhasa in Tibet is several square kilometers in size, and used to supply a large fraction of the electricity of Lhasa. At an altitude between 4,290 m (14,074.803171 ft) and 4,500 m (14,763.77955 ft), this is a strong candidate for the highest altitude set of hot springs on earth.
- Taiwan, is ranked among one of the world's top hot spring sites, harboring a great variety of springs, including hot springs, cold springs, mud springs, and seabed hot springs.
- Icaria, Greece features a radioactive hot water spring that has been used since the fourth century BCE.
- There are numerous hot springs in Greenland, such as in Uunartoq. There are over 2000 hot springs just on Disko Island, which has an area only 0.4% of that of Greenland.
- The Geysir hot springs in Iceland are the source of the word "geyser".
- The closest town to Machu Picchu in Peru is Machu Picchu Pueblo, which features several hot springs. The local name for Machu Picchu Pueblo is Aguas Calientes.
- Iceland has many famous hot springs, including the one feeding the Blue Lagoon spa in Grindavik, Iceland, and Europe's highest flow rate hot spring Deildartunguhver. Deildartunguhver's water emerges at 97 C (206.6 F) and is piped many miles to heat neighboring towns.
- Shiretoko National Park in Hokkaido, Japan has a hot springs waterfall called Kamuiwakkayu-no-taki, which translates as "river of the gods" in the Ainu language.
- Northwest Spitsbergen National Park, Spitsbergen at 80 degrees north, contains two of earth's most northerly hot springs.
- There are many geothermal springs in the UK, but the thermal springs found in the town of Bath produce the highest temperature geothermal water in the UK. The Bath hot springs are only true hot springs in the UK, by some definitions.
- Oymyakon in eastern Siberia is a candidate for the coldest permanently-inhabited location in the Northern Hemisphere and another hot springs site.  The Yakut language word "oymyakon" means "river doesn't freeze" after the local tributary of the Indigirka River fed by the hot springs which continues to flow year round in this permafrost region.
- Being located in the "Pacific Ring of Fire", Japan is in a volcanic region, and is home to many hot springs. The onsen (a Japanese word for "hot spring") plays a notable role in Japanese culture. Visiting an onsen is a quintessential Japanese experience and is a popular tourist activity.
- Chutsen Chugang Hot Springs are located on the grounds of the Zhoto Terdrom / Tidro Nunnery, at an altitude of 4400 meters in Maldrogongkar / Mozhugongka County, Lhasa, Tibet. Buddhist nuns and the "hot spring snake" both live near this set of high altitude hot springs.
- there is a hot spring on Deception Island in Antarctica.
- Champaign Hot Springs is a shallow submarine geothermal spring system along the coast of the island of Dominica, Lesser Antilles.