The Cay Sal Bank, Bahamas
The turquoise blue arch below the island is a section of the Great Bahama Bank, a submerged carbonate platform. The extreme difference in color along a razor-sharp line is due to a steep and sudden drop-off from shallow water to deep water.
Upon opening the full image, the Cay Sal Bank, an atoll of roughly triangular shape located is 145 km west of Andros Island, can also been seen towards the center. It is the third largest and the westernmost of the Bahama Banks. The dark blue waters between the two Banks form the Santaren Channel.
Again, the lighter color is due to its shallow waters: the lagoonal surface has a depth of 9 to 16 meters. Its base along the south rim is of 105 km, and its width is of 66 km north-south. It has islets along its rim, 96 in total, except along the south side facing Nicholas Channel, where it has only numerous rocky coral heads. As such, it is one of the largest atolls of the world.
In a geographical sense, it is separate from the Bahamas proper as it is much closer to Cuba (from which it is separated by Nicholas Channel, at a distance of 50 km) than to the closest Bahamanian island.
The Straits of Florida separate the Cay Sal Bank from the United States mainland and the Florida Keys (Key Largo is 100 km to the north). Some of the Florida Keys are visible in the upper left corner of the full image, surrounded by a green algal or phytoplankton bloom.