For every 100 meters you go belowground, the temperature of the rock increases about 3 degrees Celsius. Deep under the surface, water sometimes makes its way close to the hot rock and turns into boiling hot water or steam. Wells, some shallow and some miles deep, are drilled into the reservoirs and bring hot water or steam to the surface.

Geothermal energy is most abundant and more easily accessible in places like Hawaii and Alaska where the geothermal reservoirs are closer to the surface. The shifting and moving tectonic plates in these areas enable the water heated from the magma below to escape more easily to the surface.

Geothermal electricity is available virtually anywhere but is located much deeper, making retrieval less cost-effective. Geothermal energy is most heavily utilized in California and Nevada.

With geothermal energy, there are no air emissions. The only discards are salts and minerals that are discharged back into the reservoir with the excess water. To protect water supplies, reinjection takes place below the aquifers. Solids are generated at some geothermal plants, which extract minerals like silica and zinc and sell them.

Proponents of using geothermal energy believe that reinjecting water into the reservoirs will provide steam as long as the earth's core continues to heat the water. However, experts with the U.S. National Renewable Energy Laboratory (NREL) have stated that individual reservoir heat diminishes over time and that reservoirs shouldn't be expected to last longer than 30 years.

California currently has the greatest number of power plants operating on geothermal energy, the most prolific plant being The Geysers. Located north of San Francisco, The Geysers came on line in 1960, spans 30 miles and now includes 21 power plants supplying approximately 750 megawatts of electricity, or enough to supply 750,000 homes. The Geysers is the sole source of power for Sonoma, Lake, and Mendocino counties, and it also supplies a portion of the power for Marin and Napa counties.

When not used to generate electricity, the hot water can be used directly to warm greenhouses and fish farms, clean industrial laundry, and dehydrate food like onions and garlic. People in Klamath Falls, Oregon, walk on sidewalks cleared of snow using geothermal heat piped underground.