The Great Blue Hole in Belize is a massive, almost perfectly circular underwater sinkhole, approximately 300 meters (1,000 ft) across and 124 meters (400 ft) deep, located in the Lighthouse Reef Atoll, a UNESCO World Heritage site. Formed from a flooded limestone cave system from the last Ice Age, it's a premier diving destination featuring deep blue water, ancient stalactites, and diverse marine life, famously explored and popularized by Jacques Cousteau.