Inland Blue Holes
The inland blue holes of Andros are some of the most unique and mysterious aquatic caves in the world. New sites are still being found while exploring their cave systems has barely begun.
Inland blue holes have the same origin as ocean blue holes, however, they differ in many ways. Inland blue holes do not "blow" and "suck" as Ocean Blue Holes do. Therefore, diving them at particular times of the tide is not a factor. The level of water will rise and fall with the tide but no current is produced.
A unique feature of the inland blue hole is the layer of fresh water that literally sits on top of the salt water below. Debris from the surrounding trees fall into the blue hole which then decays resulting in a build-up of tannic acid and hydrogen sulfide in the water. This reduces the surface visibility to almost zero at times. As you descend through this layer of fresh/brackish water you will pass through the mixing zone or halocline into the salt water which instantly becomes crystal clear. Stalactites and beautiful flowstone can be seen in these caves which are evidence that they were once dry and above sea level.