photo from wikipedia

For divers willing to brave the long and often rough ride out to the Lighthouse Reef Atoll, a striking reward awaits. Blue holes are vertical limestone caves created from erosion and weather changes during the Ice Age. When the oceans rose, these caves were then submerged, creating watery sinkholes. The Great Blue Hole off the coast of Belize near Ambergris Caye is the largest in the world at nearly 1,000 feet across and 480 feet deep. The depth causes the water in the hole to look darker than the surrounding shallower water, resulting in its dramatic appearance. The deep, still waters hold limited sea life past a certain depth, although sharks make an occasional appearance. There, past 100 feet, the typical formations of a cave -- stalactites and stalagmites -- form a strange and beautiful landscape. The experience of diving the hole becomes similar to that of diving a horizontal cave system.