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.