The oldest source of energy on Earth: the latent heat of the Big Bang.

Geothermal energy works on the same principal as the groundsource heat pump. The energy derived from the extremely high temperature at the Earth's core can be exploited when this heat rises to the surface. There are various ways in which this can happen, and it will only occur under certain geological conditions. Geysers and hot springs are two examples where breaks in the rock allow this heat to escape, another is an underground geothermal reservoir, which can be exploited by a geothermal power plant. The hot water or steam from these sources is then used to generate electricity, and can be easily recycled by returning it to the reservoir for reheating.

And in the UK?

The number of potential sites for ground source heat pumps is much greater than that for geothermal power plants. The North Pennines, parts of southern England and Derbyshire are the only few places where geothermal sites exist where the breaks in the rock are sufficient to allow the passage of water to be heated. There is currently one geothermal plant in the UK, in Southampton.