http://earth.rice.edu/mtpe/hydro/hydrosphere/hot/freshwater/FreshWater_OCP06.html
Earth's largest aquifer, the Ogalalla Aquifer, is located under the U.S. Midwest.For more info: http://www.umac.org/ocp/freshwater/ and here:
http://www.hpwd.com/ogallala/ogallala.asp