Natural heat stored in the earth’s interior represents an essentially inexhaustible energy supply which, at usefully high temperatures, is accessible at practical drilling depths from almost anywhere on the earth’s land surface. The problems of extracting and using this heat are those of engineering and economics, and can be expected to vary with the local geology and value of thermal energy. The first major experimental system designed to investigate these problems in one common type of geologic environment has recently been completed in the crystalline rock underlying the Jemez Plateau of northern New Mexico. It consists principally of two boreholes connected at a depth of about 2.7 km by a system of hydraulic fractures produced in granitic rock at a temperature of approximately 185°C. Cool water injected through one hole is heated as it flows through the fractures, and is recovered through the second hole as pressurized, superheated water. In a surface heat-exchange system now being completed, this heat will be extracted and the cool water reinjected to maintain a continuous, closed, pressurized-water energy-extraction loop.