Most of the heat in the upper 30,000 feet (10,000 m) of the lithosphere is contained in low-permeability, more often than not dry, sedimentary and basement rocks. Access to these reservoirs is straightforward enough, and is routinely achieved (albeit for different reasons) during drilling operations in the oil and gas industry. But if the hydrothermal conditions are inadequate their viability as a productive and cost-effective energy resource is not high.