The shallows of the Lighthouse Reef in Belize appear through the crystal waters like the surface of a flooded moon. Swimmers should be warned of one hell of a drop off, though. Centered in the Half Moon Caye Natural Monument, the reef becomes a sheer limestone sinkhole - a “Great Blue Hole” in the ocean that drops 480 meters, while opening to the sides into underwater caves. A dive is only likely to be accompanied by the stalactites and a diving partner, perhaps an occasional passing sea creature, but too little light penetrates the depths for this drowned crater to maintain populations of small fish like the coral reef does. The vast (and thus relatively unexplored) regions of the caves alone draw divers. A trip to Ambergris without diving the Blue Hole is inexcusable.