Dean's Blue Hole begins one step off a beach you can walk across. You stand on white sand in knee-deep turquoise, take another pace, and the floor is simply gone: the seabed falls away in a single shaft to 202 metres. The opening at the surface is only about 30 metres wide, but below the rim the walls flare outward into a submerged chamber roughly 100 metres across, so a diver on the descent line hangs in the middle of a dark room with no visible sides. For years the hole was sold as the deepest on Earth. It is now the third deepest known, behind Mexico's Taam Ja' and the South China Sea's Dragon Hole. The shaft is older than the water in it: during the ice ages, when the ocean stood more than 100 metres lower, this was dry limestone that rainwater soaked into and slowly dissolved from within. The cave roof collapsed, the ice melted, the sea came back, and the flooded shaft was left open to the ocean.
Three sides of the cove are a curved wall of pale limestone, and that amphitheatre blocks the wind and the swell. The water inside stays flat, currentless and between 25 and 30°C all year, which is why freediving made this its home arena. William Trubridge went down 102 metres here on one breath with no fins in July 2016, a mark that stood until a Croatian, Petar Klovar, took it a metre deeper in Egypt in May 2025. Nothing here is fenced or ticketed: no gate, no lifeguard, no kiosk. Most people never leave the sandbank, standing chest-deep in warm, clear water while an arm's length away the sand slides over the edge and pours down into the dark like smoke.