A "Blue Hole" is an underwater sinkhole or "vertical cave."  Dean's Blue Hole on Long Island in the Bahamas is the deepest known blue hole, sinking 663 ft deep.  At the top it is about 100 ft across, but when you drop to a depth of 66 ft it opens up  into a 330 ft wide cavern.

I first became aware of Dean's Blue Hole when I saw an awesomely eerie video of freediver Guillaume Nery decending into the abyss (see below).  Blue holes are popular freediving destinations.  Frediving school, Vertical Blue, dubs Dean's Blue Hole "The World's Best" freediving location...

"Imagine a salt-water swimming pool, with warm, calm waters that run 203m deep, populated by snapper, jacks and colourful tropical fish, patrolled by a school of silver-plated tarpon and visited by curious rays, turtles and tiny seahorses.  An underwater cathedral festooned with light and life."

Check out the video:

via Kottke.org