Societies in the 21st century require enormous amounts of energy to drive the machines of commerce and to sustain the lifestyles that many people have come to expect. Today, most of this energy is derived from oil, natural gas, and coal, supplemented by nuclear power. Local exceptions exist, but oil is by far the most common source of energy worldwide. Oil resources, however, are nonrenewable and concentrated in only a few places around the globe, creating uncertainty in long-term supply for many nations. At the time of the Middle East oil embargo of the 1970s, about a third of the United States oil supply was imported, mostly from that region. An interruption in the flow of this import disrupted nearly every citizen’s daily life, as well as the Nation’s economy. In response, the Federal Government launched substantial programs to accelerate development of means to increasingly harness “alternative energies”—primarily biomass, geothermal, solar, and wind. The new emphasis on simultaneously pursuing development of several sources of energy recognized the timeless wisdom found in the proverb of “not putting all eggs in one basket.” This book helps explain the role that geothermal resources can play in helping promote such diversity and in satisfying our Nation’s vast energy needs as we enter a new millennium. For centuries, people have enjoyed the benefits of geothermal energy available at hot springs, but it is only through technological advances made during the 20th century that we can tap this energy source in the subsurface and use it in a variety of ways, including the generation of electricity. Geothermal resources are simply exploitable concentrations of the Earth’s natural heat (thermal energy). The Earth is a bountiful source of thermal energy, continuously producing heat at depth, primarily by the decay of naturally occurring radioactive isotopes—principally of uranium, thorium, and potassium—that occur in small amounts in all rocks. This heat then rises to and through the Earth’s surface, where it escapes into the atmosphere. The amount of heat that flows annually from the Earth into the atmosphere is enormous—equivalent to ten times the annual energy consumption of the United States and more than that needed to power all nations of the world, if it could be fully harnessed. Even if only 1 percent of the thermal energy contained within the uppermost 10 kilometers of our planet could be tapped, this amount would be 500 times that contained in all oil and gas resources of the world. How might we benefit from this vast amount of thermal energy beneath our feet? Where, by what means, and how much of the Earth’s natural heat can be usefully harnessed? These are especially important questions to contemplate, because global population is expected to soon exceed seven billion and many scientists believe that the world’s fossilfuel resources may be substantially depleted within this century. Faced with such prospects, both the public and private sectors are working toward more fully utilizing the Earth’s abundant thermal energy and other alternative energy resources. A skeptic might question the wisdom of devoting much national effort to geothermal energy development, especially because many experts think that geothermal heat can contribute at most about 10 percent to the Nation’s energy supply using current technologies. However, ongoing advances in exploration and heat-extraction technologies are improving our ability to use the resource and may substantially increase the geothermal contribution to the Nation’s energy supply. In an attempt to help national planners and average citizens alike understand the nature and energy potential of geothermal resources, this book (1) describes the distribution and nature of geothermal energy, (2) reviews the common types of geothermal systems that provide useful energy with current technology, (3) considers potential geothermal resources that might someday be tapped with developing technologies, and (4) summarizes the role of earth-science information in assessing and harnessing geothermal resources wherever they occur worldwide. The predecessor to this book (Tapping the Earth’s Natural Heat, U.S. Geological Survey Circular 1125, published in 1994) summarized the situation in the early 1990s. In an effort to support national energy planners, this new circular incorporates more recent advances in geothermal science and technology.