The Great Bahama Bank is a carbonate platform submerged in the shallow waters around Andros Island in the Bahamas. The limestone that makes up the Bank has been accumulating for at least 140 million years and today is more than 2.8 miles (4.5 km) thick. During the last ice ages, when sea level was roughly 390 feet (120 m) lower, the Great Bahama Bank was dry land.