Groundwater - A Vast Hidden Resource

Water lies beneath almost every part of the Earth's surface—mountains, plains, and deserts—but underground water is not always easy to find, and, once found, it may not be readily accessible. Groundwater may lie close to the surface, as in a marsh, or it may occur many hundreds of feet below the surface, as in some dry areas of the nation's West. The amount of groundwater lying within a half mile under the U.S. land surface is approximately four times the amount that fills the Great Lakes.

People have known about the presence of water underground since ancient times, but it is only recently that geologists have learned how to gauge the quantity of groundwater and have begun to estimate its vast potential for use. While an estimated one million cubic miles of the Earth's groundwater is located within about one-half mile of the surface (there are about one billion gallons in a cubic mile), only a small amount of this reservoir of underground water can be tapped and made available for human use through wells and springs.