Small Hope Bay Lodge Eco-Tours on Andros Island, Bahamas

The Blue Holes on Andros

Blue Holes are submarine cave systems. Their name is derived from the fact that when viewed from the air, they often have a deep blue color--deeper and bluer than the waters that surround them.

To understand their formation, one needs to go back to the beginning of the Bahama Islands themselves, back over 150 million years ago when marine sediments began to accumulate in the warm shallow seas of the tropical Atlantic. (The origins of the name Bahamas is from the Spanish words Baja Mar, meaning shallow sea). It was during this time that early corals began to colonize along the banks of sediment. These tiny corals slowly grew to form a fringing reef, which eventually solidified into limestone barriers, which in turn acted as retaining walls, trapping the sediments.

Being protected by these reef barriers, the sediment was spared from the erosion of ocean current, and was able to accumulate and sit until it too solidified into limestone. Slowly this limestone bank grew, eventually spreading out to what we now call Florida and Cuba. Subsequent erosion finally split and re-shaped the plateau, separating the Bahamas from Florida and Cuba. Rising and falling sea levels slowly carved cuts and passes until eventually the deep channels and submarine canyons began to shape the archipelago that was to become the Bahamas.

In the past two million years, the sea levels have risen and fallen in excess of four hundred feet. As water was extracted from the world oceans during each of these glacial stages, the Bahamian plateau towered above the sea. Rain water, acidic even back then, began to pool on the limestone and ran across the plateau eroding the limestone rock as it traveled. Because limestone is porous, the rain found its way through the plateau surface and began eroding an intricate maze of tunnels throughout the subsurface.

In addition to erosion by rainwater, the sea also served to shape the cave systems. As the sea levels rose with the melting of the ice cap, the tidal surges of the ocean mixed with freshwater. The solution of fresh water, seasoned with tannic acids and hydrocarbons, and mixing with salt water is more corrosive than either medium separately. The effect scoured the caves to even greater dimensions -- a process that continues to this day.

Andros Island has over 350 known inland and ocean blue holes. Inland blue Holes experience little to no discernable currents from tides. Water levels do rise in fall in response to tides, but only be indirect seepage at depths below 80-100 feet. Above the seawater, in depths from few feet to over 100 feet from the surface, lies a lens of fresh water formed by accumulating rain. The ocean sea water on each side of the island forms a hydraulic bowl, which collects the fresh water. So much water accumulates in fact, that Andros Island is able to pump off this lens on a daily basis into tankers, which is barged to Nassau providing the city with much needed drinking water.

The accumulation of freshwater above the saltwater facilitates several reactions:

• Thermoclines--a prominent and discernable separation layer where two temperatures (cool rain and warmer saltwater) mix;
• Haloclines--a discernable mixing layer where fresh water and denser salt water mix;
• Rings of Saturn--a 'smoke in the water' effect caused by the slow decay of fallen debris;
• Characteristic Hues--a discernable tint of color in the water, caused by varying concentrations of tannic acid.

Ocean Blue Holes differ from inland blue holes in that they experience huge tidal volume exchange four times a day. Often the openings of ocean holes are quite small and narrow and the water velocity can be quite extreme with ripping currents. A unique exchange of fresh and salt water occurs, as rainwater migrates into ocean blue holes with the outgoing tide. Fresh water finds its way out through the fissures and crevices in the limestone, filling the ocean blue holes to varying degrees. Some ocean blue holes experience very little or no accumulation of fresh water at low tide, while others can be almost completely full depending on the unique characteristic of each site. Tides and time continue to shape the blue holes.

One of our favorite dive sites is one we call simply "The Blue Hole" a generous and forgiving ocean hole that is part of our regular dive schedule. This blue hole demonstrates a classic Aston Collapse.

An Aston Collapse (or sinkhole) is a mammoth cave that lost part of its top. During the last ice age, the cave roof was continually eroded by fresh water seepage and tidal action. Eventually the ability of thecave's roof to support its own weight was compromised. When the cave was flooded with seawater, there was no problem--the weight of the roof was supported by hydraulic pressure. But when this support was removed as the sea levels dropped, the roof gave way to the stress of gravity. The debris created by the collapse is now scattered and wedged between the fallen roof and the existing cave walls. This lodged debris varies in depth ranging from 80 feet to in excess of 400 feet. These variations create conditions enabling us to dive the Blue Hole at various depth profiles, as we weave our way through the passages.

Small Hope Bay Lodge offers nine different Blue Hole Dives on its diving agenda. Details may be found in our Specialty Diving Section.