A hot spring is a natural body of water heated by geothermal energy beneath the Earth’s surface, emerging at a temperature above the normal human body temperature of 36.7 °C (98 °F). They range from pleasantly warm pools to violently boiling vents, and they’re found on every continent, typically clustered near the boundaries of tectonic plates.
How Hot Springs Form
Rainwater and snowmelt seep deep underground through cracks in rock, sometimes traveling thousands of feet below the surface. At depth, the water encounters rock heated by the Earth’s interior, absorbs that heat, and rises back toward the surface through natural channels. The temperature and flow rate of any given hot spring depend on how deep the water travels, how much heat it picks up along the way, and how much it mixes with cooler groundwater near the surface.
As the heated water moves through rock, it dissolves minerals. The most common dissolved minerals in hot springs are silica, calcium, calcium carbonate, magnesium, and potassium. This is why hot spring water often has a distinctive color, smell, or feel. Springs rich in sulfur compounds produce the familiar rotten-egg odor. Springs high in silica can appear a vivid blue. Calcium carbonate-rich water builds terraces of white mineral deposits over time, like those at Pamukkale in Turkey or Mammoth Hot Springs in Yellowstone.
Hot Springs vs. Geysers and Fumaroles
Hot springs, geysers, and fumaroles are all expressions of geothermal activity, but they behave differently at the surface. A hot spring forms where the water table intersects the ground, allowing heated water to flow steadily and relatively calmly. A geyser, by contrast, erupts: large amounts of superheated water fill underground cavities, and when a portion suddenly flashes into steam, the pressure violently ejects the water upward. A fumarole doesn’t discharge liquid water at all. Its underground channels pass through the water table, converting the water to steam and venting mixtures of steam and volcanic gases.
In short, hot springs flow, geysers erupt, and fumaroles steam.
Where Hot Springs Are Found
Most hot springs sit near the boundaries of tectonic plates, where the Earth’s crust is thinnest and geothermal heat reaches the surface most easily. The Ring of Fire, the arc of tectonic activity encircling the Pacific Ocean, is one of the most active geothermal zones on the planet. Countries along this belt, including Japan, New Zealand, Indonesia, Iceland, and the western United States, are loaded with thermal springs.
Hot springs also exist away from plate boundaries wherever magma chambers, volcanic remnants, or deep fault lines bring heat close to the surface. Yellowstone National Park sits over an enormous magma plume and contains more than 10,000 thermal features. Deep-ocean hot springs, called hydrothermal vents, form along mid-ocean ridges where tectonic plates spread apart. Because of the immense water pressure at those depths, ocean vents can reach temperatures of up to 400 °C (750 °F) without the water boiling.
Temperature Range
On land, hot springs span an enormous range. Some are barely warmer than a bathtub, hovering around 37 to 40 °C. Others reach a full boil at 100 °C. The springs people typically soak in for recreation fall between 37 and 42 °C, warm enough to relax muscles without posing an immediate burn risk. Above about 44 °C, water starts to feel uncomfortably hot, and prolonged exposure can cause scalding. Springs at or near boiling are dangerous and exist in places like Yellowstone, Iceland, and the Danakil Depression in East Africa.
Life in Extreme Heat
Hot springs are home to heat-loving microorganisms called thermophiles, bacteria and other single-celled organisms that thrive at temperatures lethal to most life. Researchers have isolated bacterial strains from hot springs that grow optimally at 50 to 55 °C, and certain genera survive in water as hot as 95 °C. The vivid mats of orange, yellow, and green visible in many hot springs are colonies of these organisms, layered by species according to the temperature gradient from the center of the pool to its edges.
These microbes aren’t just a curiosity. A heat-stable enzyme originally extracted from a bacterium found in a Yellowstone hot spring became the foundation for PCR technology, the same method used in DNA testing and COVID-19 diagnostics. Hot spring ecosystems continue to be a source of biotechnology applications because the proteins these organisms produce remain functional at temperatures that would destroy most biological molecules.
Health Benefits of Soaking
Bathing in naturally heated mineral water, a practice known as balneotherapy, has been used for centuries across cultures. Modern research supports some of those traditional claims. Systematic reviews have found that soaking in mineral-rich thermal water produces measurable improvements in quality of life for people with rheumatoid arthritis, with benefits appearing across multiple studies regardless of differences in how the treatment was administered. Sessions in these studies typically lasted about 20 minutes in water between 35 and 38 °C.
Beyond clinical applications, many people report that hot spring soaking relieves muscle soreness, reduces joint stiffness, and improves sleep. The combination of heat, buoyancy, and mineral content likely contributes. Warm water increases blood flow to muscles and joints, buoyancy reduces the load on painful joints, and dissolved minerals like magnesium may be absorbed through the skin in small amounts.
Safety Risks
The most obvious danger is burns. Undeveloped hot springs can reach boiling temperatures with no warning signs, and the ground surrounding them is sometimes thin and fragile. At Yellowstone, people have died after falling into superheated pools. Even springs that feel comfortable at the surface can have channels of much hotter water feeding in unpredictably.
Less obvious is the microbial risk. Human pathogens can survive and even grow in hot springs. Research using genetic sequencing of spring water has detected bacteria from the genera Legionella and Clostridium, which include species responsible for Legionnaires’ disease, botulism, and tetanus. Some springs have also tested positive for more dangerous organisms classified as high-risk pathogens. Interestingly, the hottest springs tend to harbor fewer pathogenic species, likely because extreme temperatures are hostile even to heat-tolerant bacteria. Springs in the recreational temperature range, warm but not scalding, present the greatest microbial diversity and, with it, the most potential for exposure to harmful organisms.
Keeping your head above water, avoiding swallowing spring water, and staying out of springs with open wounds are practical steps that reduce risk. Developed hot spring resorts typically test and treat their water, but wild or undeveloped springs carry more uncertainty.