The amount of heat stored in volcanic terrains is tremendous. However, there would be great difficulty in fully utilizing this energy as long as conventional heat extraction methods are applied. We propose a downhole coaxial heat exchanger system as a method of extracting geothermal energy. This heat exchanger system is applicable not only to those low-productivity geothermal reservoirs which produce insufficient amounts of steam or hot water for power generation, but also to very high temperature formations, particularly those adjacent to magma bodies or magma itself