Blue Holes

Natural Wonders: Dean’s Blue Hole is a record-setter

One of the best-known geographic features of the Out Islands is Dean’s Blue Hole. Located in a bay northwest of Clarence Town, Long Island, Dean’s is the second-deepest ocean blue hole in the world at 663 feet. Only Dragon Hole in China is known to be deeper at 987 feet.

Dean’s is also well-known as the site of the annual Vertical Blue Freediving International Competition, where numerous records have been set over the years. Enclosed on three sides by cliffs, Dean’s opens into an azure lagoon with white-sand beaches. It is completely calm, and a school of tarpon is frequently visible from the surface. If you’re truly adventurous, Vertical Blue offers freediving courses for the beginner. verticalblue.net

There are more than 1,000 blue holes in The Bahamas, of which fewer than 20 percent have been seriously explored by divers. They formed in the huge limestone plateau that makes up these islands, initially by rainwater erosion at the surface during long-ago ice ages, when sea levels were more than 300 feet lower. During those periods, numerous calcite stalactites, stalagmites, and other formations developed and are visible today to cave divers. Well below the surface, the caves usually branch out with long horizontal passages, some of which link up to other holes.

Biologist’s Dream

Blue holes come in two varieties: those connected to the ocean at the surface or deep underwater which are flushed by tidal flows and where sea life is similar to the open oceans, and inland blue holes that are fully or partially isolated from the open marine environment. Although they are less well-known, the latter is of more interest to scientists in terms of their biology, geology and chemical environment.

These contain a lens of fresh water from rainfall floating on the surface and typically extending down 30 to 60 feet. This floats on top of the denser salt water that fills the lower levels of the cave, closing it off from atmospheric oxygen. As a result, unique bacteria, fish, and other aquatic life have evolved here, which closely reflect the earliest forms of life that developed on Earth billions of years ago before the atmosphere contained oxygen.

The calcite formations are also of interest, as the layers created during growth provide a window into climatic conditions tens of thousands of years ago. There is even evidence of recent life; skeletal remains of the Lucayan people are frequently found and are well-preserved because of the oxygen-free environment.