The great Belizian Blue Hole.

Bizarre and unusual destinations around the world: "There are many blue holes off the coast of Belize, but the 'great' Blue Hole on Lighthouse Reef Atoll is the one most visible in a flyover--and the most famous one for diving. The hole is about 1,000 feet in diameter and 412 feet deep. The Blue Hole sits on the Lighthouse Reef Atoll, about 50 miles east of Belize City.

The late Jacques Cousteau and a filming crew explored the underwater cave whose roof collapsed about 10,000 years ago."