When vacationing divers dip into the upper levels of Bahamas'blue holes-flooded inland caves formed originally from limestone-to take a look around, they are unwittingly close to some of the world's most dangerous diving. Farther below lies a kingdom of passageways that holds fossils and ancient formations. The very few who pass through the layer of toxic gas to reach these lower levels find pinhole passageways where a technical failure or wrong turn could spell doom and one errant fin could obliterate 10,000-year-old rock structures. But those who do venture into the watery veins of the Earth discover whole ballrooms full of tightly packed stalactites, prehistoric human remains, and fossils of now extinct crocodiles and tortoises. Thes caves are, quite literally, another world.