Objects are smaller when they are cold, larger when hot. In geothermal district heating systems, one or more wells can serve entire districts. Electrons may be freed from atoms to produce an electric current. One of the many examples are heat energy being converted into mechanical energy, and then mechanical energy into electrical energy, as is done in steam-driven electric power plants. Energy resources are often classified as renewable or non-renewable.
Energy can take many forms, including electrical, chemical, radiant, mechanical and heat.
Geothermal reservoirs are sometimes overlain by a layer of impermeable rock. While geothermal reservoirs usually have surface manifestations such as hot springs or fumaroles, some do not. At certain levels this is beneficial because it keeps the planet warm enough for life as we know it. However, an increase in the normal amount of carbon dioxide and other gases may contribute to a human-caused warming trend that could have serious effects on global climate, the global ecosystem, and food supplies. See these terms.