Famous adventurer and filmmaker Jacques Cousteau was the first to really explore the depths of the Great Blue Hole in 1971. When he saw that its limestone cave formations featured stalactites and stalagmites, which only form above water via minerals dripping from cave ceilings, he realized that much of the Great Blue Hole had been on dry land being submerged over time as sea levels rose tens of thousands of years ago.
According to Belize.com, the aquatic wonder is 410 feet (125 meters) deep—taller than the world's tallest tree— and 984 feet (300 meters) across, which is more than three football fields placed end-to-end. It sits right in the middle of Lighthouse Reef, an eastern section of the Belize Barrier Reef, which is the one of the largest coral reef systems in the world, second only to the Great Barrier Reef in Australia. The intricate cave system, teeming with Caribbean reef sharks, is made even more awe-inspiring by the crystal-clear blue waters of the Caribbean Sea.