The Great Blue Hole is a humongous underwater sinkhole that lies just 60 miles in the Caribbean Sea off the coast of beautiful Belize. This hole, which, when seen from the top, is nearly a perfect circle, and looks so inviting that it would be all you could to resist the temptation to simply jump in.
Now, if Belize is breathtaking, the Great Blue Hole is a refreshing circle of dark indigo amidst all that turquoise water. The Great Blue Hole is part of the Lighthouse Reef System and sits just near its center. The "hole" itself measures around 300 meters in diameter and 146 meters in depth.
The Blue Hole was once a subterranean limestone cave system, but earthquakes and other geological events resulted in repeated collapses of the caves. So, what you now see is a "blue hole". You can still see the huge stalagmites and stalactites when you swim under the remains of the caves...