Geothermal energy (from the Greek roots geo, meaning earth, and thermos, meaning heat) is energy made by heat inside the Earth's crust. It's clean and sustainable.
Although the Sun does heat the surface of the Earth, heat from inside the Earth is not caused by the Sun. The geothermal energy of the Earth's crust comes 20% from the original formation of the planet, and 80% from the radioactive decay of minerals. The Earth is hottest at its core and, from the core to the surface, the temperature gets gradually cooler.
Resources of geothermal energy range from the shallow ground to hot water and hot rock found a few miles beneath the Earth's surface, and down even deeper to the extremely high temperatures of molten rock called magma. It has been used for bathing from Paleolithic times, but is now better known for making electricity.
All over the world, geothermal energy has been used to make about 10 gigawatts of electricity in 2007, and give 0.3% of the electricity needed around the world. When used to generate electricity, geothermal power plants typically offer constant output.

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