The Geysers in Sonoma and Lake Counties, Calif.

Despite having declined from a peak production of 2,000 megawatts in the mid-1980's to the present value of about 1,000 megawatts, The Geysers remains the most productive geothermal field in the world, providing nearly 60 percent of the electricity used in California's North Coast region, which stretches from the Golden Gate Bridge to the Oregon border. (The decline is due to depletion of the aquifer from which the plants draw their steam; newer plant designs re-inject the water in order to eliminate this problem.)

The first commercial geothermal power plant in the U.S. was built at The Geysers in 1960; it produced 11 megawatts of power. Individual plants at this location now average about 50 megawatts, but are dwarfed by the largest geothermal power plant currently proposed, which would be built in Sarulla, North Sumatra, Indonesia, by geothermal technology company Ormat and its partners, producing 330 megawatts of electricity at peak.