Bahamas Caves Research Foundation

What's a Blue Hole? 
Blue holes get their name due to 
the apparent blue color of their 
surface water. This is generally 
just a reflection of the sky on the 
water when viewed from an aircraft. 
The surface water in some blue holes 
can be dark, tannin or even muddy, 
but the deep water below is usually 
very clear.






The best scientific description for 

blue holes is:

"Blue holes are subsurface voids that
are developed in carbonate banks and 
islands; are open to the surface; contain
tidally influenced waters of fresh, marine 
or mixed chemistry; extend below sea level
for a majority of their depth; and may provide 
access to submerged cave passages".
(From Mylroie and Carew 1995)




Unlike the springs found in Florida, 
the underwater cave systems of the Bahamas 
(collectively called blue holes) are the 
result of several erosional processes, 
including:
·   High and low sea level stands 
   associated with glacial (ice ages) 
   and inter-glacial periods.

·   Chemical dissolution of the calcium 
    carbonate due to carbon dioxide-laden 
    rain- water percolating down through the 
    limestone.

·   Chemical dissolution of the calcium 
    carbonate due to hydrogen sulfide and 
    associated sulfur reducing bacteria within 
    in the water column. Hydrogen sulfide is a 
    byproduct of degrading organic matter that 
    is introduced into the cave from the surface, 
    or in the case of marine (completely submerged 
    sea caves) caves, from organic matter being 
    drawn into the cave on the incoming tides.
   
    OR a combination of two or more of the above 
    processes over time.

Blue holes in the Bahamas are usually classified 
as one of several types:

Geologic:

Sinkholes  

These are the large round blue holes usually seen
 from the air and are the type of formation where 
the Blue Holes get their name.

Fault line or fracture  

These very deep caves are usually associated 
with localized faulting. The systems generally 
run parallel to a deep-water, offshore canyons 
such as the Tongue of the Ocean east of 
Andros Island, or the Exuma Sound, east of 
the Exuma Cays.

Lens based blue holes  

These caves are usually the longest of the blue 
holes and are often highly decorated with 
SPELEOTHEMS. They are the result of a 
well-defined fresh water lens and the processes 
discussed above, actively dissolving a vast 
system of passages into the limestone. 
These are also known as flooded flank margin 
caves.

Geographic:

Inland blue holes  

These can be of any geological type as 
described above, with the entrance being 
accessed from land.

Marine or ocean blue holes 

These can also be of any geological type, 
with entrances being access from below 
sea level. Generally, tides and currents 
heavily influence this type of cave.

It should be noted that due to the unique 
geology of the Bahamas, each of these 
different types of caves could be 
associated with or connected to each other. 



For example, Angelfish Blue Hole in the 
Exuma Cays is predominantly a lens-based 
cave that connects through several passages 
to a major local fault line cave system. 
After their initial creation, fault line caves 
continue to enlarge by the same chemical 
processes as lens-based caves. 
Because of its subsurface entrance this 
cave is also considered a marine or ocean 
blue hole.

Another example is Bottomly's Blue Hole 
and Mystery Cave on Stocking Island, Exuma. 
Bottomly's is an inland (Anchialine) blue hole, 
that is connected by a labyrinth of passages 
to Mystery cave, which is a very tidally effected 
marine blue hole. 



For more information on Anchialine caves, 
check out Dr. Tom Iliffe's website:  
http://www.cavebiology.com/ 
This unique combination of cave types is not 
found anywhere else in the world, making 
underwater cave exploration and research in 
the Bahamas both exciting and challenging.