How it works: heat from inside the Earth is used to heat water and make it into steam. The steam turns a generator that produces electricity.
How much supply: one analysis claims that about 10% of the United States' electricity needs could be generated by geothermal energy.
Problems: the main problem is that geothermal energy is not available everywhere. There are a limited number of places where there are "hot spots" near enough to the surface to be used. Also, the technology for exploiting this resource is not yet very mature and so geothermal is not as feasible as it may one day become.