Geothermal energy uses the heat from deep underground to make electricity. It can be used to produce steam which goes up a pipe, which then pushes a turbine. It is best used in places where the Earth's crust is not real thick. In the United States, most of the western states have areas where this works. California makes the most geothermal energy. Iceland uses the most geothermal energy (per person) of any country in the world.
Once it is built, it is clean energy, but it requires deep wells. These areas often have volcanoes or earthquakes in the area, and sometimes adding or removing water deep underground might be enough to cause an earthquake. Some small earthquakes may have been caused this way.