Great Blue Hole
Today's blue holes, which are found all across the world, were formed from erosion during previous ice ages when the Earth's sea level was significantly lower than it is now.
Although the Great Blue Hole is most definitely great, it is not the deepest. That title is held by Dean's Blue Hole, located in the Bahamas, with a depth of 663 feet. For a demonstration on the magnitude of its size, check out Guillaume Nery, world champion freediver, as he does a freefall dive inside the hole in this video clip shot by fellow freediving champion, Julie Gautier.