(CNN) -- Most people have an inkling that if you drilled down to the Earth's core, it would feel pretty hot down there -- around 5,500 degrees Celsius (9,932 degrees Fahrenheit) to be precise, in fact, according to Greenpeace. (Nearer the Earth's surface, however it is a more bearable 10-16 degrees Celsius (50-60 degrees F) all year round.)

All that heat produces an extremely high amount of virtually limitless energy. Geothermal energy is the art of exploiting that very resource.

Geothermal energy is the art of exploiting the vast amounts of hot water and steam that is to be found deep within the rocks and fractures beneath the Earth's crust.

The hot water that is found nearer the Earth's surface can be directly pumped into our homes in the form of heating and hot water. And conversely, in Summer time that same system can cool buildings too.

According to the U.S. Department of Energy (DoE), when used on an industrial scale, geothermal fields emit one-sixth of the carbon dioxide (CO2) emissions that come from a "clean" natural gas power plant.

The advantage geothermal energy has over other energy sources, is it is available for use 24 hours a day, all year-round, at a rate of 90% (compared to 75% for coal), the DoE says.

According to Environmental Defense, existing geothermal plants in the U.S. produce enough electricity to power 3.5 million homes. The known geothermal reserves, however -- if utilized with currently available technology -- could power the whole nation for the next 30 years.

Geothermal energy is considered a renewable energy resource as the volume of heat or water taken out can be reinjected back into geothermal areas.

The largest complex of geothermal power plants to be found anywhere in the world are known as the Geysers, and can be found in North California.

While geothermal energy is not in widespread use, the knowledge and technology has been around for more than a century. According to Sierra Age Power, the world's very first geothermal plant began operating back in 1904 in Italy. It is still running today.

Countries around the world that currently use geothermal energy include Iceland, the U.S., Japan, Italy, Indonesia, New Zealand, Mexico, the Philippines, Kenya, Costa Rica and Nicaragua.

(Sources: Greenpeace; Calpine; Environmental Defense; U.S. Department of Energy;Sierra Age Power)