Flooded Sinkholes

Yes, ladies and gentlemen, it’s been a while since I made one of these posts. Let’s hope I’m not too rusty, and look at this;

That's right, gasp in awe!

This picture has been floating around on the internet for a long time, and it’s quite possible that many of you have seen it already, or heard of it. What you’re looking at is The Great Blue Hole of Belize, sometimes called only The Great Blue Hole.

The Great Blue Hole is by far the most famous blue hole in the world, but it is not the only one, nor the deepest, or strangest. I will present some more water-filled holes upon the face of the Earth to you, but let’s start with a little something on this one.

Jacques-Yves Cousteau made this big beauty of a hole famous through his popular tv series called “The Undersea World of Jacques Cousteau”, and he also announced the hole one of the top 4 scuba diving sites in the world. In 1971 he arrived with his ship Calypso and her crew to explore the grand abyss and chart it, and what they found was quite exciting: stalactites and stalagmites 30 meters (98 ft) below sea level, and deeper still. This of course implies that the hole was formed above sea, well, which also the ledges inside the 124 m (406 ft) cave deep implied. They also found that many of the stalactites were off-vertical by 5 degrees, which implies that the cave was once in a different angle, until some kind of geological shift moved it into the way it stands today.

However, 26 years after Jaques Cousteau went to the site, another expedition ventured into the depths to try to unravel the past of the sinkhole. The californian scientists Anthony Jones and Robert Dill led the expedition of 1997 into the hole, gathering a whole bunch of pieces of stalactites to analyse (this is apparently one of the few times when you’re allowed to damage salactites, to do isotope analysis), and they found that the cave was formed in at least four episodes 153,000; 66,000; 60,000 and 15,000 years ago, which also explains the ledges at 21, 49 and 91 meters (69, 161 and 299 ft) below the surface. At these four times in history there were climate changes and lower temperature that could have formed the Great Blue Hole indeed.

Huge stalactites inside the Blue Hole.

So, the opening is a round, circular dark hole that makes you feel like you’re going to be sucked into the depths until you simply cease to exist. Well, come to think of it, the hole is only 124 m (407 ft) deep, which means that it isn’t deeper than it is across (approximately 318 m (984 ft) ) and the lucid water makes in not all that hard to see pretty deep into the cave. And there’s also fishes living around the hole, groupers, nurse sharks and various reef sharks makes an appearance, occasionally bullsharks and hammerheads have been sighted too.

At 90-101 m (295-331 ft) below the surface however, is a strange, turbid, brown layer of water, before there’s once again lucid water below it, pretty much like above except a lot darker and with a complete lack of oxygen.
The brown layer is filled with hydrogen sulphide, which is created as organic substances sink into the hole and is decomposed by various micro-organisms. The micro-organisms deplete the oxygen at the bottom of the hole, and therefore there is nothing below the sulphide except great rocks of limestone. The sulphide makes the water more acidic and it erodes the limestone walls of the cave, which could contribute to the hourglass shape.

Anyhow, it’s not completely certain how the sinkhole was created, you need to remember that the “real” bottom of the hole is below thousands of years of sediments, and the sea level did sink in the past, but not enough for a hole of this size to be carved out. For sinkholes to be created you need a powerful flow of freshwater, which the small and lonely Lighthouse Reef Atoll would not be able to produce.

Now, there are plenty of blue holes, in the Bahamas, for example. The whole place is made out of carbonite rocks, and the seawater has hollowed out countless cave systems in the layers of stone, and most of them have yet to be discovered and explored. Those blue holes we’re talking about, however, have been eaten out by the sea from below, the black holes I’m about to tell you about however, are different. They’ve been formed by erosion from the rising and sinking sea levels, and eventually turned into big, round lakes.

There are more than 30 black holes in the Bahamas, all except one of them are on the biggest island, Andros. The other one is on Grand Bahama.

Now the black hole I’m going to tell you about it The Black Hole of South Andros. Now, I am not a scientist, and I doubt you are if you’ve found your way to this blog for some information on this black lake, so I’m going to try to keep most of the scientific gibberish out of the post, but you can find it here in case you actually are a scientist and you think my simpleton version just isn’t enough.

I will also disappoint you with a lack of images, you see, the Black Hole isn’t really any kind of tourist attraction, your biggest chance of ever seeing it is through the window of a plane, and I really can’t find you any pictures of it. It’s approximately as big across as the Blue Hole of Belize, but rather more, black, I imagine, and surrounded by land. In difference to the blue hole, diving is prohibited, and also very dangerous, and I imagine it’s not a very pleasant dive either.

Anyhow, the black holes have always been seen from the planes, but back in 1985 they were believed to be merely craters after meteorite impacts, and even though there are a lot of people in the area, no one bothered with a scientific expedition until 1999, and leading that expedition was the experienced cave diver, lawyer, and geomicrobiologist Stephanie Schwabe. The waters were tested, all the way to the bottom, revealing that there were no oxygen deep down, however, once they dived down they were surprised despite numerous tests.

At 17.7 m (58 ft) below the surface there was a strange, muddy bottom. And the lake had been measured to be 47 m (154 ft) deep. As Stephanie touched the bottom she noticed to her surprise that it was hot, and the jelly-like surface was disturbed by her touch. Would you dive into that? Me neither. But she did. She dove through this 36°C fake bottom and down into the water underneath. You see, once the jelly-like layer ended there was the same clear water as above, but in complete darkness. You might think that this is similar to the Blue Hole of Belize, and it is, except this bacterial layer was eating the light, raising the temperature and highly toxic at that.

Once the divers returned to the surface they realized that all the metal they’d worn had turned black.

Now, above the bacterial layer there’s your usual, common water. 26°C, as is common in the area, also some fish and zooplankton, at 17.7-19 m, (58-62 ft) where the ca 1 m thick layer of bacteria starts, the temperature rises to 36°C, and even 40°C. Those bacteria are called “phototrophic purple sulphur bacteria”, and to exist all they need is light and sulphur. The purple sulphur bacteria exists in other places of the world, but usually just around 5-10 m (16-32 ft) below the surface, perhaps the lucidity of the water made it favorable for the bacteria to go deeper.

There’s a lot of scientific terms around those bacteria, but I wont go into them, but there’s estimated to be 5.6 tons of dryweight bacteria in the lake. In the bacterial layer the density is 10 million living cells in one millilitre.

And the bottom layer of the lake, below the bacteria, is your common water, but with a significant lack of light and oxygen, and the temperature drops to 26°C once again. The floor of the lake is covered with purple-orange microbial mats, and many specialists believe that this might be what the ocean floor looked like 3.5 billion years ago when there was almost no oxygen in the water.

And if you made it through that big heap of text I’ll give you a little reward now. I’ll admit I was going to give you a long one about El Zacatón and Pozzo del Merro, but I doubt you’re in any condition to read it right now. So let’s head on to the little reward;

Dean's Blue Hole and a man diving in it.

Dean’s Blue Hole is another blue hole (Duh) where the room of water expands from the opening close to the surface of the water, which is approximately 25×30 m (82×98 ft), and the hole becomes larger and larger until it reaches 100 m (328 ft) in diameter (… And diafeet?). And what I really wanted to show you is this video that scares me and fascinates me.

If you’re interested in more posts like this, with less text, I direct you to The Eye of Africa and The Door to Hell.

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5 responses to “Flooded Sinkholes

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    • Thank you very much, I’m glad you like it! And it’s always nice to get some feedback too.
      I hope you have a very nice day! =)

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    • Thank you Artykuły, I’m happy to hear your opinion and I will see what I can do about the layout. Thank you very much for your insight!

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