Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Geothermal energy is thermal energy generated and stored in the Earth, which originates from the original formation of the planet (20%) and from radioactive decay of minerals (80%). Geothermal power is cost effective, reliable, sustainable, and environmentally friendly, but has historically been limited to areas near tectonic plate boundaries. Recent technological advances have dramatically expanded the range and size of viable resources, especially for applications such as home heating, opening a potential for widespread exploitation. Learn more about geothermal energy in this book.