power obtained by using heat from the Earth's interior. Most geothermal resources are in regions of active volcanism. Hot springs, geysers, pools of boiling mud, and fumaroles (vents of volcanic gases and heated groundwater) are the most easily exploited sources of such energy. The ancient Romans used hot springs to heat baths and homes, and similar uses are still found in some geothermal regions of the world, such as Iceland, Turkey, and Japan. The greatest potential for geothermal energy, however, lies in the generation of electricity. Geothermal energy was first used to produce electric power at Larderello, Italy, in 1904. By the late 20th century, geothermal power plants were in operation in Italy, New Zealand, Japan, Iceland, Mexico, the United States, and elsewhere, and many others were under construction in other countries.

The most useful geothermal resources are hot water and steam trapped in subsurface formations or reservoirs and having temperatures ranging from 176° to 662° F (80° to 350° C). Water and steam hotter than 356° F (180° C) are the most easily exploited for electric-power generation and are utilized by most existing geothermal power plants. In these plants the hot water is flashed to steam, which is then used to drive a turbine whose mechanical energy is then converted to electricity by a generator. Hot, dry subsurface rocks may also become more widely used as a source of geothermal energy once the technical problems of circulating water through them for heating and conversion to steam are completely resolved. The development of geothermal resources has become increasingly attractive owing to the rising cost of petroleum and the nonpolluting character of geothermal energy production.