Dean's Blue Hole is the world's deepest known blue hole with an entrance below the sea level. This particular blue hole is a 663-foot-deep underwater cavern, just a few steps off the beach on a remote island in the Bahamas where free divers flock -- but the locals stay away. In this video, William Trubridge breaks the world record in Unassisted Freediving with a dive to 88m (288ft) in 3 minutes 30 seconds. The dive was set at the Vertical Blue Invitational Freediving Competition.