The Great Blue Hole is a large, deep, circular sinkhole off the coast of Belize. It is one of the most popular diving destinations in the world. The hole is 305 meters (1,000 feet) in diameter and 124 meters (407 feet) deep. It was formed during the last glacial period, when sea levels were lower and the cave was exposed to air. The cave then collapsed, creating the sinkhole. The Great Blue Hole is home to a variety of marine life, including sharks, turtles, dolphins, and stingrays. The walls of the hole are covered in coral and sponges.