Blue Holes: scuba diving guide
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Many different kinds of diving spots exist throughout the world and are suited for everyone. Reefs, wrecks, caves, cenotes, ice, etc... The climate, underwater topology, and history are factors amongst others that can influence the biome and structures (natural or man-made) that can be found in each of the diving spots
In this article, I'll talk about another type of diving spot: Blue holes. Blue holes are underwater sinkholes that reveal a deeper shade of blue when seen from above due to the vertical cavity (hence the name).
Although they can be found in many parts of the world, they require some specific conditions for their formations, which I'll go over in the next section.
Blue Holes, on top of being peculiar and interesting underwater formations, are usually "touristy" spots, which makes them quite popular amongst divers.
Formation
Not every blue hole was formed the same way, but generally, they need a carbonate sedimentary base such as limestones. Said rocks are soluble and would allow underground sinkholes to be formed as a result of something known as Karst process: the dissolution of carbonate rocks.
The ones that are formed through this process may also have tunnel and cave systems present throughout their main shaft.
Blue holes can also be formed by the sudden collapse of an underwater cave, bypassing the karst process altogether.
Other mechanisms such as natural weathering and erosion also come into play. Blue holes may have formed over 100,000 years ago when the sea level was 100m+ lower than now.
Life Inside Blue Holes
The conditions inside blue holes may differ from the outer ocean as a result of their formation, which involves -as detailed earlier- multiple chemical and possible geological processes. Some of those conditions may be:
- Presence of both Fresh and Saltwater
- Anoxic past a certain depth (lack of dissolved oxygen in the water)
- Prevalence of sulphur compound
Those conditions may make it tricky for larger life forms to prosper -although fossils, including reptile fossils, have been found in a Blue Hole in the Bahamas- and will instead favor Microbial life.
Bacteria, which can adapt to many conditions, thrive and prosper in Blue holes: Even as those same conditions vary as a function of depth, bacteria adapt quickly and form sub-colonies in different sections/branches of those Blue Holes.
Risks
Blue holes are coined as being risky, and although it is not without basis, I also think the risk is quite overblown.
An issue that may arise in a Blue Hole as opposed to another spot is the possible encounter of Haloclines (a layer of water separated from another layer of water with a different salinity) which may affect your buoyancy. This is because it is possible for a Blue Hole to have fresh and saltwater at different depths. The proper control of your buoyancy is quite important in those conditions given that blue holes can be quite deep (See Yuri Lipski Incident here).
This being said, diving into a blue hole is no different from diving in any other spot if done with precautions.
Notable blue holes for diving
It wouldn't be a real article about blue holes if I didn't talk about real blue holes, so here are a couple of the most notable/famous Blue Holes in the world:
Great blue hole, Belize
The Great Blue Hole in Belize is the biggest Blue Hole in terms of total volume (taking depth and width into consideration): Only reaching a maximum depth of 124m (or 407 feet), it is its diameter that shines at a whopping 318m (1,043 feet).
IN 1971, Jacques Cousteau himself charted the depth of this Blue Hole onboard the RV Calypso and deemed it one of the five best diving spots in the whole world.
Blue Hole, Dahab, Egypt
This blue hole stands out amongst others as there have been an estimated 150-200 divers that have died there which gave it its reputation as the world's deadliest diving spot.
The Blue Hole has a maximum depth of around 100m inside and is connected to the outer ocean by an underwater tunnel known as "The Arch". The top of the arch is at a depth of 55m.
Here is the issue: the Blue Hole itself isn't inherently dangerous. However, trying to pass through the arch and reach the open ocean is: this is because it attracts many unqualified/untrained divers while being a technical dive.
Dragon's Hole, China
Dragon's Hole was thought to be the deepest blue Hole in the world -at 300.89m- until 2024, when it was found to be second to Taam Ja' Blue Hole (described further below).
As you can see in the Image below, Dragon's Hole is surrounded by shallow reef, making its access by boat complicated.
Dragon's Hole is found in the Paracel Islands, situated about 280 km/175 miles from mainland China. It is also called Yongle Blue Hole, denominated after Zhu Di, the Yongle Emperor, or the third emperor of the Ming Dynasty.
Blue Hole, Gozo, Malta
Situated on the northern Island of Gozo, the Blue Hole of Malta is one of the most popular diving spots in the country. Despite its small size, it offers plenty to see for all.
The blue hole was situated right next to the "Azure window", an arch that spanned from the sea bottom to ~10m above sea level. A severe winter storm in 2017 resulted in the collapse of this arch; this collapse left us with many swim-throughs and pinnacles, which made this dive even more famous.
The Blue Hole is also extremely famous amongst tourists and freedivers.
Taam Ja' Blue Hole
The Taam Ja' Blue Hole is the (currently known) deepest Blue Hole in the world at a staggering depth of 420m (1,380 feet)! You could fit the entire Empire State Building in the hole, with about 40m (130 feet) to spare! The hole
It is found in Chetumal Bay, which is in the Yucatan Peninsula, home to over 10,000 cenotes. Taam Ja' in Yucatec Mayan, translates to deep water.
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