Geothermal energy is a renewable energy source using heat from within the earth’s core. The heat from the earth’s core is produced by the slow decay of radioactive particles found in all rocks. This heat can be recovered as steam or hot water and can be used to heat buildings or generate electricity.

To learn more, go to:
http://tonto.eia.doe.gov/kids/energy.cfm?page=geothermal_home-basics