Blue Hole, Lighthouse Reef, Belize
Belize Blue Hole from afar
About 60 miles from Belize City, there is a perfectly circular 1,000 (305 m) feet across and 400 feet (123 m)
wide deep "Blue Hole." The hole, part of the Lighthouse Reef system, was once an opening to a cave system during the Ice Age - it is now a magnet for extreme divers:
For all the practical purposes the over 400-foot depth makes the Blue Hole a bottomless pit. The walls are sheer from the surface until a depth of approximately 110 feet where you will begin to encounter stalactite formtions which actually angle back, allowing you to dive underneath monstrous overhangs. Hovering amongst the stalactites, you can't help but feel humbled by the knowledge that the massive formation before you once stood high and dry above the surface of the sea eons ago.
Dive Belize has the story: Link