Great Lakes, chain of 5 large freshwater lakes in North America, forming the largest lake group in the world and covering an area of 95,170 sq mi (246,490 sq km). From east to west the lakes are Ontario, Erie, Huron, Michigan, and Superior. They are connected by several channels. Once important for fur trade, the lake system is used for the transportation of iron ore, steel, petroleum, coal, grain, and heavy manufactured goods. Import Great Lakes cities include Milwaukee, Chicago, Detroit, Cleveland, Buffalo, and Toronto. In recent years, the lakes, particularly Lake Erie, have suffered from serious pollution.