Dean's Blue Hole is the deepest Blue Hole in the world, and the second largest underwater chamber. It is enclosed on 3 sides by a natural rock amphitheatre, and on the fourth side by a turquoise lagoon and powder white beach. Swell and wind don't reach inside the Hole, and visibility is usually between 10 - 30 meters (30 - 100 feet). At the surface the Blue Hole is 25 x 35m (80 x 120 feet), but opens out after 20m (60 feet) into a cavern with a diameter of at least 100m (330 feet) - we still haven't found the edges!

It is still unknown how Dean's Blue Hole was formed exactly, as it is almost twice as deep as any of the other cents in the Caribbean that were formed when limestone chambers caved in from above. One hypothesis is that a much deeper cave slowed and moved upward as its ceiling eroded away.

A school of tarpon hang in the shadows at 20m (65 feet) and friendly turtles sometimes come into the hole for a break from the ocean currents. The coral caves and sand banks on the side of the entrance harbour all kinds of tropical reef life, from tiny shrimps up to groupers and snappers.