The Earth's Heat.

Inside of the Earth is a giant source of heat called geothermal power that can be used to run electrical generators. One of the first geothermal power units was built in Larderello, Italy, in the early 1900s. Until 1960 there were not any in the United States because it cost too much to get to the heat source and carry the heat to the surface in usable form.

Superheated Steam.

The Earth's crust is about twenty miles thick, but it is not the same thickness everywhere. Under the crust is magma, and cracks in the crust allow magma to come close to the surface in some places. Magma is about 10 percent water, which works its way down from the surface. When the water works its way back through cracks in the crust, it forms fumaroles — geysers with superheated steam. Normally...