The Great Blue Hole is one of the most spectacular underwater caves in the world. It lies off the coast of Belize. The big blue hole is almost perfectly circular with a diameter of 300 meters and a depth of more than 100 meters. During the last ice age, when sea level was not so high, it was a limestone cave, above the water line. Later on the roof collapsed. It is a typical example and sinkhole. The Great Blue Hole is a part of the larger Belize Barrier Reef Reserve System, a World Heritage Site of the United Nations Educational, Scientific and Cultural Organization (UNESCO).