http://www.newsweek.com/belize-great-blue-hole-long-lost-divers-found-1383300

The Great Blue Hole off the coast of Belize is the world’s biggest ocean sinkhole, measuring 1,000 feet across and over 400 feet in depth. It formed thousands of years ago as a limestone cave. As sea levels rose at the end of the last ice age, it was submerged. In the 1970s, the site was popularized by legendary explorer Jacques Cousteau. Ever since then, it has been a popular diving site—but its internal structure, features, and what lies at the very bottom remained a mystery.

In December last year, an expedition into the Great Blue Hole took place. Billionaire Richard Branson and Fabien Cousteau (the explorer's grandson) joined National Geographic Explorer and submersible pilot Erika Bergman in a dive to the bottom of the chasm.