Ever hear of a sinkhole in the middle of the ocean?

[Sankalp Anand]

Well, it happens. This natural phenomenon is found 59 miles (96
kilometers) off the coast of Belize, in the middle of the shallow
Lighthouse Reef Atoll. Evidently, about 10,000 years ago, the ocean
floor fell through to expose a cave system beneath, according to the
L.A. Times. The collapse was triggered by rising sea levels that
pressured the cave system to buckle, leaving a large vertical hole
full of water.

http://howstuffworks.files.wordpress.com/2010/10/blog-great-blue-hole1.jpg

The near-perfectly round, 412-feet-deep (125.5 meters) and 1,000-feet-
wide (304.8 meters), aptly named Great Blue Hole is so large and so
blue it can even be seen from space! (Check out a NASA photo here.)
Inside the hole, the décor is more cavelike than oceanlike. Instead of
colorful coral, scuba divers spy 40-foot stalactites in the gloom,
according to writer Jad Davenport. Aquatic life is spare, save the
sharks that loom in the dim waters — bull, hammerhead, reef and
blacktip among them. In fact, life tapers off completely near the
bottom of the hole, where there’s no oxygen, according to USGS. But
divers can typically only descend about 130 feet — to a cavern — where
they take a look at the uncommonly large stalactites, then return to
the sunny surface.