Deep down in the earth's crust, there is molten rock (magma). Molten rock is simply rocks that have melted into liquid form as a result of extreme heat under the earth. This can be found about 1800 miles deep below the surface, but closer to the surface, the rocks layers are hot enough to keep water and air spaces there at a temperature of about 50-60 degrees F (10-16 degrees C). Geothermal technology takes advantage of the hot close-to-earth-surface temperatures to generate power.