Geothermal energy is subsurface Earth heat that flows to the surface from hot water and/or steam reservoirs or heated rock formations, through a variety of pathways. Heat is carried continuously upward to the Earth's surface as steam or hot water when water flows through permeable rock. There are a variety of applications for geothermal energy and the technologies for extracting thermal energy also differs widely. Over the years, technologies have evolved that allow the extraction of this heat. The geothermal energy potential in the uppermost 6 miles of the Earth's mantle amounts to 50,000 times the energy of all oil and gas resources in the world. Geothermal resources offer a large source of secure energy to the nation's energy portfolio. Geothermal production can help minimize the need for petroleum imports, reducing the international trade deficit and providing jobs. As of 1990, there were 8,147.8 MW of geothermal generating capacity installed worldwide. Of these, 2,850 MW are installed in the United States, and 5,297.8 MW are installed in other countries, primarily the Philippines, Mexico, Italy, Japan, and New Zealand.