Andros Island Blue Holes.....

Diving an Ocean Blue Hole

Blue Holes are unique geological formations created over thousands of years and multiple glacial and interglacial periods.

There are many types of Blue Holes, but they are essentially caves that formed in a air environment when sea levels were low as a result of  glacial retreat. When the ice retreated the caves flooded and this process of being dry and flooded was repeated several times over as the glaciers came and went.

In Land\Ocean Blue Hole Conch Sound Andros Island

As the caves tops grew thinner from erosion or experienced mechanical stress many of them collapsed leaving large holes often leading to hidden cave systems. These holes got the name Blue Holes due to the fact that the bottoms of the collapse is deeper than surrounding land and when viewed from the air appear rich blue in colour in the ocean, some are dark blue and even black in appearance inland. The inland Blue Holes collect a less dense freshwater lens on top of the ground seawater and are connected to the ocean Blue Holes by cracks and fissures typical of the limestone composition. Imagine a sponge.

Diving an Ocean Blue Hole

The tide moves seawater back and forth between the inland Blue holes and the ocean Blue holes via these cracks and fissures during the high and low cycles of the tide.

Often the Blue holes have strong inflow and out flows as water is exchanged. The cooler hypoxic/anoxic inland water (caused by oxygen dissolving into the limestone)flows out of the ocean Blue holes and this mixes with the warmer sea water. Where the two meet it creates a mixture of haloclines, chemoclines and thermoclines often referred to as a "density interface"

Each Blue Hole has its own personality and offer an opportunity for divers to experience a glimpse of the past geology and formations. Proper training is a must to dive these beautiful structures in safety. Even the cavern portions require experienced guides.

Check out the August edition of National Geographic Magazine for more great facts and pictures.

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