INTRODUCTION Conventional geothermal technology entails the production of useful energy from natural sources of steam or, much more commonly, hot water. These hydrothermal resources are found in a number of locations around the world, but they are the exception rather than the rule. In most places, the earth grows hotter with increasing depth, but mobile water is absent. The vast majority of the world’s accessible geothermal energy is found in rock that is hot but essentially dry -the so-called hot dry rock (HDR) resource. The total amount of heat contained in HDR at accessible depths has been estimated to be on the order of 10 billion quads (a quad is the energy equivalent of about 180 million barrels of oil and 90 quads represents the total US energy consumption in 2001). This is about 800 times greater than the estimated energy content of all hydrothermal resources and 300 times greater than the fossil fuel resource base that includes all petroleum, natural gas, and coal. (Tester, et al. 1989). Like hydrothermal energy resources already being commercially extracted, HDR holds the promise for being an environmentally clean energy resource, particularly with regard to carbon dioxide emissions, which can be expected to be practically zero. The total HDR resource base noted above was calculated by summing the thermal energy content of rock beneath the landmasses of the world at temperatures above 25C (77F), from the surface to a depth of 30,000 ft (9,150 m). Obviously, much of this HDR resource resides in rock that is only marginally warmer than 25C and is thus of such low-grade that it is not practical to recover it. In addition, a large part of the resource may be located in parts of the world where its exploitation may not be economically worthwhile. Nevertheless, with such a large resource base, the potential for HDR to be a major contributor to the world’s energy supply makes its development well worth pursuing, especially when considered in light of its environmental advantages. One method of evaluating the potential for HDR development in a region is to examine its geothermal gradient -the rate at which the earth gets hotter with depth. The geothermal gradient varies widely from place to place, being much higher in tectonically active regions and in areas of volcanic activity. Figure 1 shows a geothermal gradient map of the United States. It is apparent from this map that HDR resources at useful temperatures (above 100C) are abundant in many parts of the west.