A geothermal pool is a naturally occurring body of water heated by the Earth’s internal energy. Groundwater seeps deep into the crust, encounters hot rock or magma, and rises back to the surface through cracks and fissures as heated water. The result is a pool that can range from pleasantly warm to near-boiling, often rich in dissolved minerals and home to some of the most unusual life forms on the planet.
How Geothermal Pools Form
The process starts underground. Rainwater and snowmelt percolate downward through porous rock, sometimes traveling thousands of feet below the surface. In volcanic regions, that water comes into contact with rock heated directly by magma chambers. In non-volcanic areas, the water simply travels deep enough that the natural increase in temperature with depth, known as the geothermal gradient, heats it significantly. On average, rock temperature rises about 25 to 30°C for every kilometer of depth.
Once heated, the water becomes less dense and begins rising back toward the surface through faults, fractures, and fissures in the rock. When it reaches the surface and collects in a depression, you get a geothermal pool. The same basic mechanism also produces geysers, steam vents (fumaroles), and bubbling mud pits. The difference comes down to how the water interacts with the surface: a pool collects and sits relatively calmly, while a geyser builds pressure underground and erupts periodically.
Temperature and Chemistry
Geothermal pools span a wide temperature range. Some sit at a comfortable 40°C (104°F), while others reach 97°C (207°F), just below boiling at sea level. The pools that people soak in recreationally are typically at the cooler end of that spectrum, but many wild geothermal pools are dangerously hot. At Yellowstone National Park, water temperatures in some features exceed 90°C, and falling into one can be fatal within seconds.
The chemistry of these pools depends on the rocks the water passes through and the gases it picks up along the way. Many geothermal pools contain high concentrations of silica, sulfur, carbon dioxide, and hydrogen sulfide (the compound responsible for the rotten-egg smell common at hot springs). Some pools also accumulate trace amounts of mercury and selenium in their sediments. The pH varies dramatically: alkaline pools fed by deep chloride-rich water typically have pH values between 6.7 and 9.5, and their water often appears a vivid blue. Acidic pools, where volcanic gases like hydrogen sulfide oxidize into sulfuric acid, can drop to a pH of 3 or lower. Mud pots are a telltale sign of an acidic system.
Life in Extreme Heat
Geothermal pools might look inhospitable, but they support thriving communities of microorganisms called thermophiles and hyperthermophiles, organisms that not only tolerate extreme heat but require it. The vivid orange, yellow, and green bands you see around the edges of many hot springs are mats of heat-loving bacteria. These microbes are among the oldest types of life on Earth, and their study has reshaped scientists’ understanding of how life can survive in extreme environments.
Bacteria dominate these ecosystems in terms of diversity. Research in geothermal fields in Peru identified over 120 distinct bacterial types in pool sediments, alongside more than 50 types of archaea, a separate domain of life that often thrives in extreme conditions. Some of these organisms, like species of Chloroflexus, can survive at temperatures up to 70°C. Others, including species of Meiothermus and Fervidobacterium, are specifically adapted to geothermal environments and are rarely found elsewhere. Some pools even harbor microbes never previously documented in thermal settings, which is part of why geothermal pools remain a focus of biological research.
Geothermal Pools vs. Hot Springs
The terms “geothermal pool” and “hot spring” overlap significantly, and in casual use they’re often interchangeable. Both describe naturally heated water that reaches the surface. The distinction, when one is drawn, is mostly about form. A hot spring typically refers to any place where geothermally heated water emerges from the ground. A geothermal pool specifically describes a collected body of that water sitting in a basin or depression. In other words, every geothermal pool is a hot spring feature, but not every hot spring forms a pool. Some hot springs flow as streams or seeps without pooling.
Other geothermal features share the same heat source but behave differently. Geysers periodically erupt because constrictions in their underground plumbing allow pressure to build. Fumaroles are vents that release steam and gas but little or no liquid water. Mud pots form when acidic water and steam break down surrounding rock into clay, creating thick, bubbling mud.
Health Benefits of Geothermal Bathing
Soaking in geothermal water, sometimes called balneotherapy, has a long therapeutic history and a growing body of clinical evidence behind it. The mineral content of the water appears to be the key factor. Salty and sulfur-rich waters in particular have keratolytic effects (they help shed dead skin cells), promote skin regeneration, and act as antioxidants. Bathing in mineral-rich geothermal water has been shown to reduce inflammation, improve microcirculation in the skin, and help regulate immune responses.
Skin conditions respond especially well. Psoriasis and atopic dermatitis (eczema) are among the most frequently and successfully treated conditions through geothermal bathing. Other conditions that benefit include acne, rosacea, and chronic itching. Beyond the skin, the warmth itself increases the flexibility of collagen-rich tissues like tendons and ligaments, which is why geothermal soaking is commonly used alongside treatment for rheumatic and musculoskeletal conditions. The heat also triggers the skin to release natural opioid-like compounds, which can raise the pain threshold and provide short-term relief.
Geothermal Pools as an Energy Source
The same underground heat that creates geothermal pools has been harnessed for practical energy use for over a century. In the 1890s, a businessman in Boise, Idaho, tapped into a local hot spring to create a bathing resort. That modest start evolved into the largest municipally operated geothermal heating utility in the United States. Today, Boise’s system includes more than 20 miles of pipeline delivering naturally heated water at 177°F to warm over six million square feet of building space, including City Hall, the local YMCA, and a public recreation pool.
Klamath Falls, Oregon, runs a similar system that has served 23 commercial and government facilities since the early 1990s, tapping a reservoir between 200 and 220°F. One of its notable uses is melting snow from sidewalks and bridges. Universities have adopted the concept too: Ball State University operates the largest ground-source closed-loop geothermal system in the country. In Washington, D.C., a pilot project announced in 2023 aims to replace fossil fuel heating at a residential development with geothermal district heating. These systems demonstrate how the same geological forces that create a scenic hot spring can also replace natural gas furnaces.
Where Geothermal Pools Are Found
Geothermal pools cluster along tectonic plate boundaries and in regions with volcanic activity. Iceland, New Zealand, Japan, and the western United States are home to some of the most famous examples. Yellowstone National Park alone contains roughly 10,000 geothermal features, including some of the most studied pools on Earth. But geothermal pools also exist in less obvious locations. Any area where water can reach sufficiently hot rock, even without active volcanoes, can produce them. The geothermal gradient ensures that rock temperature increases with depth everywhere on the planet; the question is simply whether water has a pathway to circulate deep enough and return to the surface.
If you visit a wild geothermal pool, the most important thing to know is that water temperature and chemistry are unpredictable. A pool that looks calm and inviting can be at or near boiling, and acidic pools can cause chemical burns. Designated soaking areas at developed hot springs are tested and managed for safe temperatures, but unmarked pools in volcanic regions should be treated with serious caution. The ground near them can also be unstable, with thin crusts over scalding water or mud just below the surface.