The Great Blue Hole is a large underwater sinkhole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll 60 miles (100 kilometers) from the mainland of Belize City. The hole is circular in shape, over 984 feet (300 meters) across and 410 feet (125 meters) deep. The world's largest natural formation of its kind, the Great Blue Hole is part of the larger Barrier Reef Reserve System, a World Heritage Site of the United Nations Educational, Scientific and Cultural Organization (UNESCO).

The hole was forged out of solid rock as caverns, hundreds of thousands of years ago during the last ice age. For a very long period of time, water filtered through the rock and into these great stone cathedrals, breeding multicolored stalactites and stalagmites. Then, one by one they collapsed in on each other, creating a chasm four hundred feet deep. As Earth's swollen polar ice-caps receded, the warm Caribbean ocean swept in to fill the chasm with boiling white hands, the last rays of sunlight struck the chasm's floor. Then, as the limestone broke down, it began to rain tiny fragments of rock, which slowly began to fill the great Blue Hole.

Recent evidence suggests that the Great Blue Hole may hold the secret to Mayan collapse as layers of sediment in the hole's floor offer scientists a geological timeline to examine. Not only did the chemical composition of the silt indicate periods of sparse rainfall during the Mayan decline (likely between AD800 and AD1000): It also showed that a second huge drought probably occurred between AD1000 and AD1100—right around the time the Mayans' relocation site of Chichen Itza is said to have fallen.