Dean's Blue Hole is a shaft in a cove on Long Island, in the Bahamas, and everything it shows you happens in the first few metres. You wade out over a sandbank in water that reaches your knees, take one more pace, and the floor is gone: the seabed drops away in a single shaft to 202 metres. Below the neck the walls flare out into a room roughly a hundred metres wide, so a diver on the line hangs in the middle of it with no sides in view. The hole is older than the sea that fills it. Rain cut it out of dry limestone during the ice ages, when the ocean stood more than a hundred metres lower. Slightly acidic water bites hardest where fresh meets salt, so the cave was hollowed from the inside long before any roof came down.
Pale limestone curves round three sides of the cove, so neither wind nor swell reaches the surface and the water stays flat, currentless and between 25 and 30 degrees all year. That is why freediving made the place its home arena. A weighted line hangs from a float in the middle — the same rope the sport's deepest dives have used. For about ten days each July a competition runs on it, with spectators treading water in a ring while a diver comes back up from beyond a hundred metres. The hard part is the last few seconds: within fifteen of surfacing the diver must pull off the mask, sign OK and say the words, and only then do the judges raise a white card. The rope has its other history too. In November 2013 the American diver Nicholas Mevoli died here after an attempt, the first death at a sanctioned competition, and it forced the sport into a hard argument about how far one breath should be pushed. Nothing here is fenced or ticketed, and almost everyone stays on the sandbank. From there you put your mask in the water, watch loose sand spill over the ledge, and watch someone go down the line and stop being visible.