This type of energy production taps into the extremely high temperatures found deep within Earth's crust, at the core. Temperatures there -- driven by natural, ongoing processes of radioactive decay -- are hotter than the surface of the sun, and we see signs of it in such geological formations as volcanoes, hot springs and geysers. To harness this heat -- sometimes as hot water, other times as steam -- we need only dig down deep enough in the right spot, where pockets of this geothermal energy are creeping to the surface. These wells provide taps into the Earth's intense heat energy.
Once the steam or hot water is collected from the well, it's easy enough to use it directly as a heating source or to spin a turbine to generate electricity. Start-up and consumer costs are relatively low, but the potential for this type of energy production is very site-specific, limited to the "geothermal reservoirs" that dot the globe. In the United States, for instance, geothermal sites are mostly in the West, and California is the largest geothermal producer. One other caveat: This type of energy may not be strictly renewable, since we could potentially use this heat faster than the Earth can replenish it.
Sunlight, on the other hand, is a tough thing to deplete …