Dean's Blue Hole is a shaft in a cove on Long Island in the Bahamas where the seabed drops 202 metres one step off a beach of wadeable sand, the deepest known blue hole in the Atlantic and the place where the world records of freediving have been set on a rope hung from a float, and it is snorkelled around its rim and swum over its blue centre in water that is glassy, currentless and warm all year. The hole was dissolved out of the limestone when the ocean stood far lower, in the ice ages, when slightly acidic rainwater corroded the rock hardest where fresh water met salt and ate a cave from the inside before the roof fell in, and the sea flooded it as the ice melted; the cove is walled by limestone cliffs that keep off the wind and the swell, so the surface stays flat, and wading off the sandbank and dropping the mask below the surface, the pale floor gives out at the rim and sand drains over the ledge into a blue that goes black. The shaft widens below the neck into a chamber a hundred metres across, and it was described for years as the deepest blue hole anywhere until deeper ones were sounded in China and Mexico.
The weighted line drops from the float in the middle, and it is the same rope on which the sport's deepest dives have been made, since the freediver William Trubridge settled on the island and holds the competition Vertical Blue here over about ten days each July, when spectators tread water in a ring around the line while a diver comes back up from beyond a hundred metres on one breath. The rest of the year the cove is quiet, with a few snorkellers and the freedivers training. Dean's is a hole in the sea floor at the edge of a beach, and the deepest breath-holds on Earth have been taken in it.