The earth's crust is heated by the decay of radioactive elements. The heat is carried by magma or water beneath the earth's surface. Some of the heat reaches the surface and manifests itself in geysers and hot springs throughout the world.

Geothermal power can be used to directly heat buildings. Further, the pressurized steam from superheated water beneath the earth's surface can be used to power turbines and thus generate electricity.

Although geothermal power seems ideal in that it is naturally occurring and does not require structures to trap or collect the energy (as in solar panels or windmills), it does have limitations. The greatest drawback is that naturally occurring geothermal vents are not widely available. Artificial vents have been successfully drilled in the ground to reach the hot rocks below and then injected with water for the production of steam. However, oftentimes the source of heat is far too deep for this method to work well.

Nor can geothermal power realistically generate enough electricity for the entire United States or any other large industrialized nation. A good-sized hot spring can power at most a moderate sized city of around 50,000 people. An there just aren't enough viable hot springs to power all the cities in any large country.