Western Bahama Banks#

Image courtesy of NASA., under PD
The green-turquoise water over the banks is less than 9 m (30 ft) deep but the deep blue of the Tongue is 1,200 to 1,800 m (4,000 to 6,000 ft) deep.
All the sediment on the banks, including the material that forms the islands, is calcium carbonate (lime) precipitated from sea water by animals and plants.