The Blue Hole is a geological oddity, so much so that in March of 1996 it was declared a World Heritage Site and later declared a National Monument in February of 1999. The Hole used to be, once upon a time millions of years ago, a complex system of dry caves. Scientists believe there were a couple of peculiar events that made the Hole what it is today. First, an earthquake of such force it might have tilted Lighthouse Reef, the area where the Blue Hole is located, to an angle of 12 degrees. Secondly, the melting of the last Ice Age flooded the cave system. Eventually, the porous limestone ceilings of the caves became incapable of supporting their own weight and they crumbled, leaving an almost perfectly round and deep hole in the process.
The Blue Hole is almost 1000 feet in diameter and over 450 feet deep. Its walls are almost perfectly vertical and fairly smooth, except at a few points where there are large ledges and overhangs. It is here that we find enormous stalactites (hanging down), stalagmites (building up) and columns (when stalactites and stalagmites meet) dating from the Pleistocene period. Due to the earthquake mentioned above, some stalactites hang at a 12-degree angle, cluing scientists such an event happened since stalactites cannot form except in a perfectly perpendicular manner. Some formations that happened after the earthquake are indeed perpendicular, and in some of the stalactites that formed before the earthquake one can see the top parts being at an angle and their bottom parts, which kept forming afterwards, being perpendicular.
Jacques Cousteau made the Blue Hole famous in 1972, when he took his famous research vessel, the Calypso, into Lighthouse Atoll and traced a route that is used by dive boats to this day. In fact, it is erroneously often mentioned that Philippe Cousteau, his son, was killed during this trip. However, that happened while he was operating a light-wing plane in Lisbon, Portugal a few years later.