Blue Holes

Documentary: Blue Holes – Diving the Labyrinth

Blue Holes
Blue Holes

Cecil, cave diver to be (he probably is one by now thanks to Buks Potgieter of IANTD) lent us this National Geographic production. It’s just under an hour long, and recounts an expedition to seven of the blue holes in the Bahamas. A blue hole is a kind of vertical cave, roughly circular. They often contain freshwater (light) on top of saltwater (heavy). The reaction between the two layers of water where they meet (called the halocline) eats away horizontal passages off the main shaft of the sinkhole over time. Sometimes there is a layer of hydrogen sulphide, created by bacteria and decaying organic matter, beneath which is an anoxic (devoid of oxygen) layer of water which preserves organic remains (bones, not flesh) with remarkable fidelity. For this reason blue holes are sometimes referred to as time capsules. The deep water is usually very, very clear.

Here’s a picture of the Blue Hole on Gozo in Malta, where Tony and I dived earlier this year. It doesn’t have a freshwater layer – it’s sea water all the way, but the top view you can see here illustrates what a blue hole looks like. Beautiful!

The Blue Hole on Gozo
The Blue Hole on Gozo

The multidisciplinary research team shown in this special aimed to uncover the ancient history of the Bahamas, which no longer have any large predators (such as crocodiles) occuring naturally there. Aside from the diving (more on that just now) there’s a bit of palaentology, anthropology and archaeology to keep history buffs happy. Blue holes are currently the subject of much reasearch, and as the narration in this special points out, rising sea levels threaten to flood the holes and destroy the anoxic conditions that make them such excellent preservers of the past – so there is a sense of urgency to the studies being conducted.

The diving, though, is mainly what interested us (and Cecil, I suspect). The footage inside the blue holes shows how beautiful they can be, with stalagmites and stalagtites in the side passages carved out by the halocline as the sea level changed. It also shows how treacherous cave diving can be – fine silt covers most of the surfaces, and even the pressure wave created by a diver swimming forward above the bottom of the cave can stir it up… Never mind a careless fin stroke or bumping the bottom!

Some of the diving was done on air and other gas mixes, using open circuit scuba with side mount arrangements to make the divers’ profiles wider but flatter. Other dives, to greater depths (both vertical and horizontal), were done on rebreathers. One of the advantages of a rebreather, apart from ridiculously long bottom time, is no bubbles, which is great for photography and for cave diving where bubbles can disturb sediment and even bring down bits of rock from the ceiling. One of the divers claims that 10% of all rebreathers sold have killed their owners (this is an exaggeration – according to DAN, only about 5% of diving fatalities are on rebreathers but one must adjust for the fact that open circuit is far more prevalent)… And that while she’s putting her kit together she thinks about all her dead friends who perished using these devices. If that was my state of mind before going in the water, I wouldn’t dive – the depression would kill me first! The divers go through a 40 point written checklist before they get in the water for a dive and are very, very careful about maintaining their rebreathers.

This is a beautiful look at the treasures hidden in places that many of us will never visit. The photography is wonderful, even if the narration is a little bit over the top dramatic (but we’ve been spoiled by the refined dignity of the BBC, I think!). It’s a little bit sobering from a diving perspective, but one can only admire people whose determination to break new ground and advance science leads them to the very edge of what technology can currently assist with.

There are some still photos from the special here (go and look – very lovely). An article on blue holes in the Bahamas appears here.

You can get a copy of the DVD here.

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Lapsed mathematician, creator of order, formulator of hypotheses. Lover of the ocean, being outdoors, the bush, reading, photography, travelling (especially in Africa) and road trips.

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