The Great Blue Hole, Belize

The Great Blue Hole is a marine sinkhole near the centre of Lighthouse Reef, an atoll 70 km off Belize City. It is almost perfectly circular, 318 metres across and 124 metres deep, covering 70,650 square metres of open water, and it is part of the Belize Barrier Reef Reserve System, a UNESCO World Heritage Site.

It was not made by the sea. It is a limestone cave that formed in dry air during the Quaternary glaciations, when sea level stood far lower than it does now, and analysis of its stalactites dates that formation to four separate episodes: 153,000, 66,000, 60,000 and 15,000 years ago. When the ice melted and the ocean came back up, the cave flooded and the roof fell in.

Jacques Cousteau made it famous. He brought the Calypso here in 1971 to chart the depths and afterwards called it one of the top five scuba diving sites in the world. That expedition confirmed the karst origin and mapped ledges at 21, 49 and 91 metres, each marking a stand of sea level.

One detail from those stalactites is easy to miss and hard to forget. Some of them hang five degrees off vertical, all leaning the same way. Stalactites grow straight down, so the rock beneath them must have tilted after they formed, and then stayed put long enough for the rest to grow true.

The ledges Cousteau charted are what divers go down for, and they are deep. This is a serious dive rather than a snorkel, and most people see the hole properly for the first time from the air, where the circle against the pale reef is unmistakable.