Sinkholes: Dean's Blue Hole

Dean's Blue Hole is located in the Bahamas. It is the deepest saltwater blue hole known in the world. A blue hole is a sinkhole filled with water that is entered below sea level. The hole is 663 feet deep. There are some freshwater holes even deeper than Dean's Blue Hole. The Hole is a popular diving destination.

 http://en.wikipedia.org/wiki/Sinkhole#Notable_examples

http://school.verticalblue.net/deans-blue-hole/