As the above quote demonstrates, the vision of utilizing the earth’s internal heat to benet the world is not new. It is obvious to even the most casual observer that energy, in the form of heat, is present below the earth’s surface-volcanoes spew scorching lava from their summits and water in hot springs bubbles up from below-attesting to the fact that the interior of the earth is hot. But the nature of that heat remained, for many years, a source of mystery. Why volcanoes are unevenly distributed over the continents and why hot springs are abundant in some places and nonexistent in others seemed inexplicable. But, over the last 200 years, intensive research by cadres of earth scientists has provided a clearer understanding of the source, distribution, and properties of this massive thermal reservoir.