The Earth has a very thin crust of bedrock, stones, sand and soil that is typically about 20 miles thick. Below the crust are layers of magma, molten rock and stone several thousand miles thick. Below the magma layers are the molten metal core of the Earth.
The enormous stored heat of the magma layers is constantly moving upward through the crust. At the surface of the Earth, air temperature and weather typically determine the temperature of the soil for at least a few inches or feet. Several feet below the surface, however, soil temperature remains constant, thanks to heat from within the Earth.
