What is a source of geothermal power?

Geothermal power means to use the heat of the Earth's interior. Earth gets this heat from three sources: (1) A residue of Earth's formation (this part is insignificant, since Earth has had time enough to cool down); (2) Radioactive decay inside the Earth; and (3) The gradual solidification of Earth's interior, since converting a liquid to a solid usually releases heat.