Geothermal energy is harnessed from the heat that has been trapped underneath the Earth's crust. Some of this heat escapes naturally in the form of volcanic eruptions or geysers, but it can also be captured manually. The heat is used to generate steam, which operates turbines to power a generator. This type of energy can also be used on a small scale to heat homes.
Geothermal accounts for 0.4% of the energy generated in the U.S.