Black and Blue Holes of Andros

Green gems scattered across the Bahamian turquoise sea, the archipelago that is Andros is made up of around 700 islets and cays connected via estuaries and tidal swamplands. As one island unit, Andros totals at about 2,300 square miles, or roughly the size of Delaware. Lush and unspoiled, Andros is home to multiple rich and varied ecosystems, the largest national park system of the Bahamas, and, famously, the most blue holes per square mile in the world.

Blue holes, while common throughout the islands of the Bahamas, are famed in Andros due to their numbers and unique features. The island of Andros can claim 178 blue holes on land, and another 50 in the sea. This peppering of blue holes makes the island unit a veritable paradise for adventurous divers keen on plunging into these mysterious blue portals.

Blue holes are entrances to sea cave systems that weave and interconnect underneath the island to the seafloor. They are named so because of their deep blue appearance, caused by the reflectivity of the water and the white carbonate deposits along the walls of the holes. Being interconnected to the inland, these blue holes can change their appearance and depth based on the tides. During higher tides, cold sub-surface water will rise, carrying with it algae and hydrogen sulfide. This will also change the appearance of the hole, sometimes causing the water to look opaque or milky.

Despite the inherent dangers of cave diving, many of these holes have become tourist destinations for adventurous travelers. One such hole, King Kong’s Blue Hole, features what is essentially an “ocean river.” This underwater river is produced by the differences in temperature and density of cool water beneath the island and the warm seawater which surrounds it. The colder, darker, and denser water sinks downward and emerges out of the stream beds from the blue hole, and forms a flow that is clearly visible against the warmer and lighter water which surrounds it.

King Kong’s blue hole is not, by any means, the only blue hole to boast amazing sights. River beds and waterfalls from ice ages, fault lines, untouched geologic features, and intricate cave systems draw in tourists from all over. Still, less than 1% of these blue hole systems are thought to have been explored, and what little exploration has been done has only occurred in recent years.

Adding to their intrigue, the blue holes of Andros are surrounded by local legend. Inhabitants of the island have long claimed that deep in the dark blue depths of the holes lives a great monster known as the Lusca. According to legend, this fearsome sea creature is said to have pointed, jagged teeth, much like a shark, with a huge squid-like body that totals in length at an enormous 75 feet. Of course, no concrete evidence has ever been found of the Lusca, but some speculate that the cave systems could easily support a giant cephalopod of some sort, and, as giant squid were also once thought to be legend, many are hopeful that they might one day discover evidence of the Lusca. However, even the Lusca, real or imaginary, might fail to be this island’s most interesting feature.

The island of Andros is home to not only blue holes, with which many of us are familiar, but also black holes. And no, we haven’t yet left for space — though, as alien as these holes are, you may believe otherwise. Black holes appear black (as opposed to their dark blue cousins) due to the presence of a toxic microbial layer. This layer of microbes is so dark that, at a glance, it might appear to be the hole’s bottom. This black layer functions as a separation, with oxygenated water above the microbes and water that is entirely devoid of free oxygen below, much like the oxygen-free ancient oceans of billions of years ago. This environment is invaluable to scientists seeking to better understand the earth’s ancient oceans.

Black holes are unusual in their formation because they develop from the top down and have no outlet to the sea. Additionally, the separation of the two water types in the column is extremely unusual. But the most alien aspect of the black hole is the microbes themselves.

Allocromatium and Thiocapsa are anoxygenic purple spirillum that inhabit the black hole of Andros. These phototrophic bacteria are capable of turning light energy into heat energy with such efficiency that they are able to raise the temperature of the water significantly, allowing them to out-compete non-thermotolerant microbes.

At about 18–19m, these purple sulfur bacteria are found in a layer about 1m in thickness, and at this depth, dramatic temperature changes occur. During a June 1999 survey, it was found that at a depth of 18m, the microbial layer maintained a distinct increase in temperature from 29 C to 36 C, tapering off after an additional meter. Some other research into black holes has shown an increase of up to 41 C attributed to purple sulfur bacteria.

Essentially, the large production of heat as an adaptation occurs from very inefficient photosynthetic reactions, which result in large amounts of heat waste. At the division of water types, there is an increase in salinity and a decrease in oxygen. Sulfate-reducers can grow in this environment and generate large amounts of hydrogen sulfide, which encourages the growth of sulfur-oxidizing bacteria. These bacteria are very specialized and thus have little competition, as this environment is rich in sulfide and saline, but relatively poor in light. Chemical processes then occur inside of the phototrophic bacteria to produce massive amounts of heat. Dr. Schwabe of the Rob Palmer Blue Holes Foundation and her peers estimate that at 17.8m, it would take about 21 days for the sulfur-oxidizing bacteria to raise the temperature of the water by 21 C.

Dr. Schwabe hopes to further black hole research in the future by exploring holes that are currently inaccessible due to un-routable terrain or the narrow dimensions of the holes themselves. Through their study, more information may be gained on “underground water sources, pollution control, and hydrological relations with limestone formation.”

Black holes and blue holes, formed by chemical erosion, offer an insight into geologic formations of the ice age as well as the ancient waters of the earth, and are examples of the extreme adaptations life can develop when left undisturbed. The island of Andros contains a wealth of information, the majority of which has yet to be uncovered. Fortunately, access to many of these sites is very restricted so as to limit contamination and destruction of these sensitive ecosystems.

The island of Andros, with its fathoms-deep trenches, legendary sea creatures, and black and blue holes, stirs awake the adventurer inside us all. Whether or not these holes house monsters or just more mysteries, they already delve into the realm of science-fiction. These holes function as time machines, allowing a look into the earth’s past, and, an opportunity to understand the almost alien world of extremophile bacteria. And, while risky, who among us could resist diving through a time portal?


“Exploring the Black Holes of Andros.” LICOR Environmental Newsline. N.p., 31 Mar. 2011. Web. 10 Nov. 2019

Herbert, R.A., Gall, A., Maoka, T., Cogdell, R.J., Robert, B., Takaichi, S., and Schwabe, S. (2008). Phototrophic purple sulfur bacteria used as heat engines in the South Andros black hole. Dordrecht.

“The Atlantic Ocean — Black Hole of Andros.” BBC. N.p., n.d. Web. 10 Nov. 2019.

“The Mysterious Blue Holes of the Bahamas.” Mysterious Universe. N.p., n.d. Web. 19 Nov. 2019.

“Ocean Blue Holes.” Small Hope. N.p., N.d. Web. 19 Nov. 2019.



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