Kern County is located in the southern Central Valley of the U.S. state of California. The county seat is Bakersfield (since 1874). The county has a large agricultural base and is a significant producer of oil, natural gas, hydro-electric power, wind-turbine power, and geothermal power. The county accounts for one-tenth of overall U.S. oil production, and three of the five largest U.S. oil fields are in Kern County. The American Lung Association ranked Kern County as the most ozone-polluted county in the US in 2006. The county has a total area of 8,161 mi², 99.7% of it is land and 0.3% of it is water.