Heat reservoirs in the form of hot rock far beneath Earth's surface are a potential source of usable geothermal energy. A. Heat reservoirs with a temperature from 80℃ to 180℃ can be used, as in France and Iceland, to heat buildings. B. Most heat reservoirs with a temperature above 180℃ cannot be used for energy because they are usually too close to recent volcanic activity. C. Experiments are under way to determine if geothermally heated waters could be used as a source of certain minerals that have been dissolved out of hot rocks deep within Earth. D. A number of countries now use geothermal reservoirs that contain water or steam above 180℃ to generate electricity. E. The sinking of land above heat reservoirs and other environmental problems arise when water is pumped into a heat reservoir under high pressure. F. A number of issues, including how to extract heat from reservoirs that do not have a natural supply of water, will significantly limit the use of geothermal energy for the foreseeable future.