Environmental activists have an aboveground and a below-ground view of the world. Energy sources harnessed on or very close to the surface, like wind, wave, tidal, solar and hydro power, are good. They are renewable and do not emit carbon dioxide, the main greenhouse gas that is widely thought to be responsible for global warming.
However, energy sources found under the ground, such as coal, oil and natural gas, and uranium used for nuclear power, are bad. Fossil fuels are major greenhouse- gas emitters while nuclear power, although it produces almost no global warming emissions, is still regarded by many environmentalists as too much of a safety and proliferation risk.
Yet there is another form of underground energy gaining increasing prominence as a future source of power that gets an environmental seal of approval: geothermal heat. What comes out of the ground with this form of energy are hot water and steam, and almost no pollution.