A blue hole is essentially formed when a roof collapses over a cavern and is subsequently filled with water. Geological structures in the Great Blue Hole of Belize are important because they initially can only form in air, when caverns are dry and above sea level. Evidence for the maximum lowering of sea level during successive ice ages over the past several millions of years is sparse. Due to its impressive appearance and mystique the Great Blue Hole was established as a World Heritage Site by the United Nations.