Only 1 percent of the water on earth is available for human uses, and this water needs special treatment to ensure its safety for drinking. Although water is the most abundant substance on earth, covering about three quarters of the earth's surface, most of it is not usable or readily available. About 97 percent is found in the oceans and is too salty for human use. Two percent is frozen in polar ice caps. Only 1 percent is liquid fresh water, found in groundwater, lakes, rivers, and wetlands, that can readily serve as a source of drinking water. Scientists have estimated that as much as 90 percent of the liquid fresh water on earth is found in the ground. Much of this water was unavailable to us until we developed the technology to drill deep wells.