Geothermics, science pertaining to Earth’s interior heat. Its main practical application is in finding natural concentrations of hot water, the source of geothermal energy, for use in electric power generation and direct heat applications such as space heating and industrial drying processes. Heat is produced within the crust and upper mantle of the Earth primarily by decay of radioactive elements. This geothermal energy is transferred to Earth's surface by diffusion and by convection movement of magma (molten rock) and deep-lying circulating water. Surface hydrothermal manifestations include hot springs, geysers, and fumaroles. Hot springs have been used since ancient times for therapy and for recreational purposes. Early Norse settlers in Iceland brought water from nearby hot springs into their shelters by means of wooden conduits.

Steam produced from the naturally hot fluid that exists in geothermal systems is an alternative to pressurized steam produced in power plants by burning fossil fuels, by utilizing nuclear energy, or by other means. Modern drilling in geothermal systems reaches concentrations of water and steam, heated by much deeper magma, at depths up to 3,000 m (10,000 ft). Steam is purified at the wellhead before being transported in large, insulated pipes to turbines.

Geothermal energy was developed for electrical power in 1904 in Tuscany (Toscana), Italy, where power production continues today. The Geysers in northern California in the United States has become the world’s largest geothermal power complex. Geothermal fluids have also been used to heat groups of buildings in many places around the world, including Budapest, Hungary; a Paris suburb; all of Reykjavík and other Icelandic cities; most of Klamath Falls, Oregon; and (since 1890) part of Boise, Idaho.