The Great Lakes system is usually considered to be the largest (95,000 square miles) source of fresh surface water in the world, with the exception of the polar ice caps. Lake Michigan is the third largest of all the Great Lakes in area and it is the largest body of fresh water entirely within the boundaries of the United States. As a natural resource of immeasurable value, Lake Michigan serves a broad spectrum of purposes. It provides drinking water for an estimated five million residents in the northeastern Illinois area. Its recreational opportunities attract fishing, boating, swimming and other water-oriented interests. The lake also serves as an avenue for domestic and international water-borne commerce. Industry and commerce have grown and developed giving the greater Chicago metropolitan area one of the largest industrial-commercial complexes in the United States. Consequently, some of Lake Michigan's water quality problems result from discharges of industrial and municipal wastes.