The Great Blue Hole is a navy circle three hundred metres across in the pale shallows of Belize's barrier reef, seventy kilometres offshore, and it is not a gap in the coral but a cave with its roof gone. Through the ice ages, with the sea far lower, this was a dry cavern growing stalactites in open air. The ice melted, the water rose, the roof let go, and the sea filled the shaft to 124 metres. The stalactites are still hanging. You drop down the sheer inner wall to about forty metres to reach them, fingers of stone over ten metres long jutting out of the gloom. Caribbean reef sharks cruise past in the blue while you hang there beside them. There is almost no coral at that depth and almost no colour; what you come down for is not a reef but the feel of a cave.
From a small plane over Lighthouse Reef it finally holds together: a dark ring in pale water with a thin rim of coral around it, and only from up there does the size settle. The living part is the rim. In two or three metres of water over the flats, southern stingrays cross the seagrass and turtles graze it, untroubled by a snorkeller overhead. Beside them the wall drops away into blue under your fins. Out at Half Moon and Long Caye, sea fans, sponges and grunts fill the same shallow water; the hole itself is the first stop of a long day, not the whole of it. The boats cross most reliably between April and June, once the winter northers have blown out and the water above the shaft clears. Deeper down it only empties. Near ninety metres the water crosses into hydrogen sulfide, and below that there is no oxygen at all. When submersibles reached the floor in 2018 they found it scattered with conch shells that had drifted in over the rim and, with nothing alive down there to rot them, never decayed.