Blue holes are, simply put, underwater caves.
They are part of an extensive cave system that was formed in the Bahamas in prehistoric times. The Bahamas platform is composed of limestone, which suffers from various erosion processes. One of these processes is continually occurring as the mild acid in rainwater causes small fissures and holes in the limestone. During the Pleistocene era a number of ice ages caused significant changes in sea levels, another of the erosion processes. The landscape formation that we see in the Bahamas today is a result of this erosion and is referred to as karst topography.
The findings of stalactites and stalagmites, which can only form in air, in blue holes confirms the long periods of lower sea-levels these caves must have endured, according to Thomas. M. Illfe, author of Anchialine Caves and cave Fauna of the World.
Types of Blue Holes
It is commonly accepted today that there are 3 types of blue holes, geologically speaking.
The following descriptions of the types of blue hole are cited from the Bahamas Caves Research Foundation website:
- Sinkholes - Large round caves that can be seen from the air. These are where blue holes get their name from; when viewed from above the dark blue of the deep water in the holes contrasts with the lighter shallow water surrounding them.
- Fault Line or Fracture - These caves are very deep and usually associated with a fault line in the local area. Generally, the fault line caves can be found running parallel to a deep off-shore water canyon e.g. the Tongue of the Ocean found to the west of the island of Andros.
- Lens Based - Rather than being the deepest these caves are usually the longest of the 3 types of blue hole. They are also referred to as flooded flank margin caves as a well-defined fresh water lens actively dissolving a system of passages into the limestone forms them.
Tidal Pull and Blue Holes
Geographically, there are two types of blue holes, inland and ocean. Inland blue holes can be accessed from land and ocean blue holes are accessed from below sea level. As these caves are so extensive, often the exit of them cannot be located. However, a blue hole accessed from land will usually have an exit that is submerged and vie versa. It is this connection between inland and submerged caves that cause the unique tidal pull that can be experienced when diving and exploring blue holes.When the tide is rising the water from the ocean will be pushed into an ocean blue hole, the water level in the connecting inland blue hole will therefore rise also as the water has nowhere else to go. When the tide change to an outgoing tide, this pressure is released and the water that has flooded into the inland blue hole will now flow back out to the ocean.
Myths and Legends of Blue Holes
There are many myths and legends to be heard in the Bahamas and there is one that relates to the blue holes.
Some blue holes will have particularly strong tidal pulls, so much so that on an outgoing tide, the surface of the water above a marine blue hole will give the effect of water boiling up out of the hole. Also, on a rising tide, divers should not explore blue holes as the pull of water into the hole can be too strong to swim against.
Before the tidal pull could explain the boiling of water and the sucking effect of the blue holes that has led to the wreckage of many fishermen and their boats, Lusca was deemed responsible.
Lusca is a fantastic creature, a giant squid with the jaws of a shark, who lives deep in the blue holes. Although we now know the tidal pull can lead to the disappearance of many, Lusca has not managed to claim all his victims and there are eyewitnesses who have managed to survive his attack and describe his terrifying appearance!
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