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