The shallow turquoise-colored banks and deep, dark blue-colored channels of the northern Bahamas are vividly captured in this photograph taken from the Space Shuttle. The shallow Great Bahama Banks, generally less than 10 feet (3 meters), plunge into the Tongue of the Ocean, a partially enclosed basin that is greater than 6500 feet (2000 meters) deep. Complicated current patterns are displayed around Andros Island and the south end of the Tongue of the Ocean. The Bahamas are composed mainly of limestone and coral. The islands, like Andros Island, are generally low and flat, with many mangrove swamps, brackish lakes, coral reefs, and shoals.