Another way of generating electricity relies on the heat energy of the core of the earth. The earth's core reaches temperatures of up to 5000 degrees Celsius. This is in part left over heat from the origin of the planet, but is even more due to the decay of radioactive elements deep in the earth's core. This tremendous heat energy melts rock, which begins to rise. Most spectacularly, if this magma bursts through to the earth's surface, we see a volcano (here in LEGO by Bryce McGlone and Brandon Griffith).
If the magma does not burst through to the surface, but instead heats up groundwater, the resulting superheated steam can burst out in the form of a geyser (here in LEGO by Brother Steven).
Geothermal power takes advantage of this superheated groundwater. Either the steam that is created directly turns a turbine, or its heat is used to vaporize a secondary liquid that then turns a turbine. In either case, this then generates electricity. MyGreenHosting made 'a LEGO set we'd like to see': geothermal powered data center. MyGreenHosting is an Icelandic web hosting company that promotes itself as environmentally friendly because they are completely powered by geothermal and hydroelectric energy.
Geothermal electricity is efficient and nonpolluting. Unfortunately, it is very limited in application because it requires very specific geological conditions. For instance, it only accounts for 0.3% of US power. Iceland is the Saudi Arabia of geothermal, with 30% of their electricity coming from geothermal. The Phillipines, El Salvador, Costa Rica, Kenya, Nicaragua and New Zealand are other countries with at least 10% of their electricity coming from geothermal.