The inner core of the Earth reaches a maximum temperature of about 4000ºC, with the outward heat flow maintained predominantly by natural radioactive decay of certain dispersed elements (e.g. uranium, thorium and certain isotopes of potassium). Heat passes out through the solid submarine and land surface mostly by conduction – geothermal heat – and occasionally by active convective currents of molten magma or heated water. The average geothermal heat flow at the Earth’s surface is only 0.06 W/m2, with average temperature gradient of 25 to 30ºC/km. This continuous heat current is trivial compared with other renewable supplies in the above surface environment that in total average about 500 W/m2 (see Fig. 1.2).However, at certain specific locations increased temperature gradients occur, indicating significant geothermal resources. Regions of geothermal potential generally have permeable rock of area ~10 sq km and depth ~5 km through which water may circulate. Consequently, they can be harnessed at fluxes of 10 to 20 W/m2 to produce ~100 MW (thermal) per km2 in commercial supplies for at least 20 years of operation. Regions of ‘hot, dry rock’ have to be fractured artificially to become permeable, so that water may be circulated through the fractures to extract the heat.