Hot water and steam from deep underground can be used to drive turbines: this is called geothermal energy.
Geothermal and nuclear energy are the only energy resources that do not come from the Sun.
Several types of rock contain radioactive substances such as uranium.
Radioactive decay of these substances releases heat energy, which warms up the rocks.
In volcanic areas, the rocks may heat water so that it rises to the surface naturally as hot water and steam.
Here the steam can be used to drive turbines and electricity generators.
Geothermal power stations are located in places such as Iceland, California and Italy.
In some places, the rocks are hot, but no hot water or steam rises to the surface.
In this situation, deep wells can be drilled down to the hot rocks and cold water pumped down.
The water runs through fractures in the rocks and is heated up.
It returns to the surface as hot water and steam, where its energy can be used to drive turbines and electricity generators.
The diagram shows how this works.