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 asIceland, Turkey, and Japan. The greatest potential for geothermal energy, however, lies in the generationof electricity. Geothermal energy was first used to produce electric power at Larderello, Italy, in 1904. Bythe 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