Enormous amount of heat energy at high temperature is locked up inside the earth. It remains confined by the earth's crust, which is a good heat insulator, but escapes at the edges of tectonic plates in the form of volcanic eruptions, hot springs and geysers. The energy coming out in the form of hot springs and geysers has been used for heating and bathing and is now being used for generation of electricity also. Invention of geothermal heat pumps has greatly increased the scope of using this energy and it is now being used in more than 70 countries.