The search for new energy resources as alternatives to fossil fuels have generated new interest in the heat of the earth itself. New geothermal areas with a variety of characteristics are being explored, as are new ways of extracting work from naturally heated steam and hot water. Some of this effort is discussed in this three-part module. Five types of geothermal resources and their development and utilization are reviewed in the first section. Geothermal energy and related principles are discussed in the second section. These include: force, work, energy, power; absolute temperature and pressure; heat engines, carnot engine; enthalpy; and availability and maximum work. Geothermal energy utilization is discussed in the third section, focusing on electricity generated from and non-electrical applications of geothermal energy. Also included in the module are study questions (with answers), a list of references, and, in appendices, measurement tables and tables of properties of saturated steam and saturated water (temperature, pressure) and properties of superheated steam and compressed water (temperature and pressure). The module contains the equivalent of 1-week's material and is suitable for use in introductory college physics courses and in advanced high school physics programs. (JN)