Geothermal electric systems generate electric power using heat from within the Earth. Engineers drill a mile or more into the Earth's crust, tapping into underground reservoirs where hot rocks and water are heated by magma deep beneath the surface. Geothermal power is sometimes called hydrothermal generation, because the energy comes from hot water. The hot steam pushes a turbine to spin a generator, generating clean electricity.
Because the Earth's core is so hot, the potential for geothermal energy is tremendous. However, geothermal reservoirs ("hot spots") are most common where the plates that make up the Earth's crust intersect. Although the United States is the world leader in total geothermal power production, all of its geothermal power is concentrated in just four western states. A single site north of San Francisco called "The Geysers" accounts for over half of all U.S. geothermal electricity. Small nations that are located on plate boundaries, such as Iceland and the Phillipines, generate large portions of their electricity from geothermal power.
Current geothermal installations have been limited to places where water is easily accessed. Engineered geothermal systems (EGS) are created by drilling into hot, dry rocks and injecting cold water under pressure. As the pressure builds, the rocks fracture and the liquids are heated by the surrounding rock. A recent study by MIT estimated that engineered geothermal systems could meet the world's electricity needs for thousands of years.