Great Blue Hole

The Great Blue Hole is a natural monument declared in 1996 in the centre of Lighthouse Reef atoll, some seventy kilometres off the coast of Belize, and is one of the seven components of the Belize Barrier Reef Reserve System inscribed that same year on the World Heritage List.

It is an almost perfectly circular marine sinkhole some three hundred metres across and more than a hundred deep, whose dark blue contrasts spectacularly with the turquoise of the shallows around it, to the point that it is recognisable from the air and from space.

Its origin is karstic and terrestrial: during the Quaternary glaciations, when sea level was far lower, the area was dry land and rainwater dissolved the limestone to form a cave system. The roof of the main chamber eventually collapsed and created a doline, and as sea level rose with the melting ice it was flooded. Underwater expeditions have documented stalactites several metres long on its walls, tilted from the vertical, which can only have formed in the dry and which also record later tectonic movements. Jacques Cousteau explored and popularised it in 1971, and today it is one of the best-known dive sites in the world.