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The Geysers, a geothermal power field located 72 miles (116 km) north of San Francisco, California, is the largest geothermal development in the world. It is currently outputting over 750 MW. The Geysers consists of 22 separate power plants that utilize steam from more than 350 producing wells.  The Calpine Corporation operates and owns 19 of the 22 facilities. The other three facilities are operated by the Northern California Power Agency and the Western GeoPower Corporation.
The Geysers geothermal development spans an area of around 30 square miles (78 km²) in Sonoma and Lake counties in California, located in the Mayacamas Mountains. Power from The Geysers provides electricity to Sonoma, Lake, Mendocino, Marin, and Napa counties. It is estimated that the development meets 60 percent of the power demand for the coastal region between the Golden Gate Bridge and the Oregon state line.
Steam used at The Geysers is produced from a greywacke sandstone reservoir, that is capped by a heterogeneous mix of low permeability rocks and underlaid by a Felsite intrusion. Gravity and seismic studies suggest that the source of heat for the steam reservoir is a large magma chamber over 4 miles (7 km) beneath the ground, and greater than 8 miles (14 km) in diameter. 
Unlike most geothermal resources, the Geysers is a dry steam field, which means it mainly produces superheated steam. Because the power plant turbines require a vapor phase input, dry steam resources are generally preferable. Otherwise, a two-phase separator is required between the turbine and the geothermal wells to remove condensation that is produced with the steam.
The first recorded discovery of The Geysers was in 1847 during John Fremont‘s survey of the Sierra Mountains and the Great Basin by William Bell Elliot. Elliot called the area “The Geysers,” although the geothermal features he discovered were not technically geysers, but fumaroles. Soon after, in 1852, The Geysers was developed into a spa for The Geysers Resort Hotel, which attracted the likes of Ulysses S. Grant, Theodore Roosevelt, and Mark Twain.
The Geysers electrical plant reached peak production in 1987, at that time serving 1.8 million people. Since then, the steam field has been in gradual decline as its underground water source decreases. Currently, the Geysers produce enough electricity for 1.1 million people.
Techniques developed from Enhanced Geothermal Systems research will increase the production of the region in the future. By reinjecting greywater from the nearby city of Santa Rosa, existing wells will be recharged. This water will be naturally heated in the geothermal reservoir, and be captured by the existing power plants as steam. The project should increase electrical output by 85 MW, enough for about 85,000 homes.