Hot Spring
Hot Spring, or Thermal Spring, a spring that discharges water warmer than normal body temperature. Springs issuing tepid water at less than body temperature are usually called warm springs. Hot springs are found in many parts of the world, including cold areas; they are most common in regions of present or past volcanic activity.
The water of some hot springs comes from molten rock material cooling and solidifying in the earth's crust. The water of most hot springs, however, is surface water that has seeped underground and then returned to the earth's surface. The water from some hot springs, such as those in Yellowstone National Park, is heated by very hot rock of volcanic origin lying relatively close to the earth's surface. The water from other hot springs, such as those of Hot Springs, Arkansas, is warmed deep below the surface by heat from the earth's interior. (In general, the temperature in the earth's crust increases with depth at the rate of about one degree F. for every 60 feet of depth, or one degree C. for every 33 m.)
Some hot springs, especially those with mineral water believed to have medicinal value, are the sites of health spas and hotels.