well as the decay of naturally occurring radioactive elements. Temperatures below the surface can reach 4,200° C, decreasing to 650 - 1200° at depths of 80km-100km. Through the deep circulation of groundwater and the intrusion of molten magma into the earth's crust at depths of only 1km-5km, the heat is brought closer to the earth's surface. The molten rock then heats the surrounding groundwater, which is forced to the surface in certain areas in the form of steam or hot water (i.e. hot springs, geysers). The heat energy close to or at the earth's surface can be utilized as a source of energy.