Blue Holes.

Blue holes
Blue holes are giant and sudden drops in underwater elevation
that get their name from the dark and foreboding blue tone they exhibit when viewed from above in relationship to surrounding waters. They can be hundreds of feet deep and while divers are able to explore some of them they are largely devoid of oxygen that would support sea life due to poor water circulation - leaving them eerily empty. Some blue holes, however, contain ancient fossil remains that have been discovered, preserved