Peridotite is a rock that's believed to make up the Earth's mantle. http://edition.cnn.com/2017/04/07/asia/japan-drill-mantle/ The mantle makes up more than 80% of the Earth's mass, lying 6 miles (10 kilometers) beneath the ocean floor.
Iceland says its geothermal power station at Reykjanes can produce clean energy independent from fossil fuels by generating electricity from the heat stored inside the earth