Geothermal energy comes the radioactive decay of metals in the earth's core, 4,000 miles underground. Near the edge of tectonic plates, magma is able to reach depths much closer to the surface, where it heats rocks (and where it exists, water) to very high temperatures, creating regions of underground steam, or hot dry rock.

Coming soon.

• The Last 10,000 Years: Geothermal springs have been used for warmth, cooking, and bathing.
• 1830: In Hot Springs, Arkansas, Asa Thompson charges a dollar for a bath in geothermal spring water - the first known commercial use of geothermal energy.
• 1847: William Bell Elliot discovers steaming valley north of San Francisco, which he calls the Geysers. He believes he has found the gates of Hell.
• 1904: First geothermal power plant is built in Tuscany, where steam is erupting from the earth.

References: EERE Geothermal History in the U.S.

• Dry Steam Plants: steam from underground is used to directly drive steam turbines to generate electricity.
• Flash Steam Plants: high pressure hot water is converted to steam to drive steam turbines to generate electricity. When it condenses, it is injected back into the ground to be recycled. Most geothermal is flash geothermal.
• Binary Plants: heat from geothermal water is transferred to a separate working fluid, which evaporates, and drives a steam turbine to generate electricity.
• Hot Dry Rock: theoretically, water cold be injected into hot dry rock to extract the heat. This requires deep drilling, and is still being developed.