Oceanic Sinkhole
The Great Blue Hole, located in the gulf of Belize, is a massive marine sinkhole. A sinkhole is a pit or hole in the earth created by a collapse in the surface layer; the sea was far away when the Great Blue Hole was developed. The cave was inundated when the sea began to rise again.
It was rated one of the top scuba diving spots in the world once it was identified. However, there was something about it that made it seem enigmatic.