Has been explored zillions of times.. A friend I graduated with is a boat captain and explores the Caribbean a lot and has sent photos of several other places called the Blue Hole.. The reason they name them the blue hole is because generally they are in shallow water, which is aqua, and its a perfect circle of dark blue, which is how deep water looks, so a hole in shallow water that goes deep would loo like a Blue Hole..
He's free dove down a 100 feet, or so, on a couple of them, but my guess is the majority of them have not been explored by cave certified divers.. but knowing cave certified divers, I can tell you its only a matter of time.. I guess any location where the bottom drops out and an underground cavity is "made available" to humans is considered a Blue Hole.. There is also one in New Mexico inland that my wife has seen while on vacation that we intend on diving, one of these days..
Friendship is like peeing your pants.. Everybody can see it, but only you can feel the warmth..