Heat from the Earth, or geothermal — geo (Earth) + thermal (heat) — energy can be harnessed at different depths for applications ranging from generating electricity to heating and cooling buildings. Geothermal resources accounted for about 5 percent of U.S. renewable energy use last year – an amount comparable to wind.

Geothermal resources range from shallow ground to hot water and rock several miles below the Earth's surface, and even farther down to the extremely hot molten rock called magma. Mile-or-more-deep wells can be drilled into underground reservoirs to tap steam and very hot water that can be brought to the surface for use in a variety of applications. In the United States, most geothermal reservoirs are located in the western states, Alaska, and Hawaii.