According to the United States Census Bureau, the city has a total area of 24.8 square miles (64 km2), of which 22.9 square miles (59 km2) is land and 2.0 square miles (5.2 km2) (7.85%) is water. The city's elevation above sea level is 176 m 186 m (577 ft 610 ft). The city sits within the boundaries of the former Lake Chicago. Most of the city is on sandy soil with a layer of black topsoil that varies from non-existent to several feet (a meter or more) thick. Much of the exposed sand has been removed for purposes such as industrial use to make concrete and glass.