The Great Blue Sinkhole: A Diver's Worst Nightmare

Have you ever gone scuba diving? A tank strapped to your back, mask over your eyes, coasting through the water marvelling at everything mother nature has to offer. The feeling as you slowly adjust to the freezing cold ocean. The unnatural phenomenon of being able to breath underwater. When you think about it, diving is very strange.

And yet there’s one place on earth where this experience gets even stranger. In the middle of a shallow atoll there is a hole that seems to dive to the centre of the earth. People used to call it bottomless, but if you ever made it all the way down there, you’d most likely be dead.

How did this geographical oddity come to be? How does a place with such stunning beauty, also represent one of the most dangerous places to dive in the world?

The largest underwater sinkhole in the world. A diver’s greatest dream and their worst nightmare all rolled into one.

The Great Blue Hole.

Introduction

The Great Blue Hole is a sinkhole located in the Caribbean Sea, approximately 70 kilometres off the coast of Belize. It has a diameter of over 300 metres (or 984 feet) it’s largest of its kind and it’s almost perfectly circular. It exists among a shallow atoll and coral reef known as Lighthouse Reef. The water level around the sinkhole is so low that parts of the rock underneath crest the surface at low tide. It’s very distinctive, as the turquoise water of the atoll makes the deep blue of the sinkhole stand out.

It’s not just wide though, it’s also deep, with it being a staggering 125 metres (or 400 feet) from the water’s surface all the way to the sea floor, roughly the same height as the Great Pyramid of Giza. Until the 1960s it was widely believed by many to be bottomless and there simply wasn’t the diving technology to find out. Getting to the sinkhole to experience it for yourself though can be quite the challenge, it’s not exactly close to shore, meaning you need to charter a boat for a few hours or catch a plane in order to get there.

People throughout the year make this journey to Belize and then from Belize to the Great Blue Hole. What are they going there to do?

Naturally, they’re going diving.

The World’s Greatest Dive

Of course, the thing that attracts people to this massive sinkhole is for its diving industry. Why you ask? Because it’s perfect for recreational divers. It’s deep and, being surrounded by a coral reef, the currents are minimal.

The weather is temperate and the water at depth stays at the same temperature for most of the year. The water is also extremely clear, with visibility on the best days reaching down to 200 feet. Diving here allows people to get up close and personal with the hole’s main attraction, its huge stalagmites and stalactites, which can reach 9-12 metres, or 30-40 feet, in length. Diving deep is reserved for experienced divers, however, anyone is able to complete a shallower dive, as well as explore the reef.

The absolute depth limit is 45 metres for recreational divers, as we’ll see later, going any deeper has proved fatal even for those with plenty of experience.

It is certainly a popular place for people to plunge, although others are less impressed. The quality of the dive is linked to several factors like weather, wildlife, and visibility, leading some to brand the Great Blue Hole as “overrated”. Some travel blogs state that the expectation of this place as a diver’s paradise can cloud people’s perceptions, especially if they were expecting a tropical paradise full of abundant sea life.

Speaking of which, what life exists in the Great Blue Hole? Well, not as much as you might think. Owing to the fact it’s a very deep sinkhole surrounded by a very shallow atoll, not much oxygenated water circulates deep down, and nor does light. This means that the deeper you go, the more life begins to disappear. It certainly gives off an uncanny valley feeling, like you’re diving into the vacuum of space. However, there is marine life here.

Angelfish, butterflyfish and small groupers are all pretty common and it’s not rare to see some species of sharks and rays in the shallower areas, as well as some coral and marine life on the walls of the sinkhole before it descends. But is there something more sinister lurking in the deep?

In the 60s and 70’s, reports of several diving expeditions into the Great Blue Hole stated that divers claimed to have witnessed an enormous sea serpent. The description of the beast is very vivid, purportedly over 20 feet long, with red eyes, semi-transparent and eel shaped, with very long dorsal and pectoral fins. The only fish that fits such a description is the giant oarfish, otherwise known as the doomsday fish, so named as it only tends to be found just before natural disasters occur.

But this is unlikely. Oarfish tend to live hundreds of metres beneath the surface, much deeper than the Great Blue Hole reaches. There could be a small chance one swam into shallower waters if it was injured or sick, but considering no oarfish have been seen there since, it’s either likely to be a hoax or an extraordinary coincidence.

Either way, the supposed existence of a huge sea monster in the Great Blue Hole adds to its mystique. In fact, one billionaire was so enthralled by the sinkhole, he decided to answer its questions once and for all. In 2018, a team of divers from Aquatica Submarines went on an expedition to map it. The expedition was fully funded by Virgin founder, space coloniser, and activist, Richard Branson.

They didn’t find much, except an empty two-litre bottle of Coca Cola and a lost GoPro camera, showing that there is quite literally nowhere on earth that us humans won’t dump our trash. The airline owner described the expedition as “one of the starkest reminders of the danger of climate change that I have ever seen.”

But what about the past? Where exactly did this huge sinkhole come from? Well to answer that we need to go back, way back.

Origins and History – From the Ice Age to Modern Day

So how did this sinkhole come to be? Well for that we need to take a look at the geology. It is believed that the sinkhole was originally formed as a limestone cavern that existed on the earth’s surface during the last ice age, approximately 150,000-15,000 years ago, when sea levels were typically much lower. The stalactites and stalagmites in the cave, support this theory, as they’re commonly found in limestone caves today.

It is also believed that an ancient earthquake caused the ceiling to collapse, creating the sinkhole structure, then as the ice age melted, sea levels rose and filled it.

The Great Blue Hole hit the mainstream in 1971 when it was made famous by intrepid explorer and renowned conservationist Jacques Cousteau. Not the pink panther, the other one. With his ship, the Calypso, he went on to explore the sinkhole through the use of his mini submarine. After investigating the depths, Cousteau confirmed that the Great Blue Hole had indeed formed from a limestone cavern and the route he pioneered to get there is still used by tourism and diving companies to this day.

Some believe that Jacques’ son, Philippe Cousteau, lost his life in the Great Blue Hole during the voyage and while this is verifiably untrue, that doesn’t mean that nobody has died there. In fact, due to a substantial number of factors, the Great Blue Hole appears to be one of the most dangerous places a person can dive recreationally or professionally. But what are they? Let’s just see what dangers this sinkhole really holds.

Sink or Swim – The Dangers of the Great Blue Hole

So, what can we expect to come across when we’re diving deep into the Great Blue Hole? Is it really more dangerous than other waterways? Well on the surface, as well as beneath it, the answer is yes. For starters, you’re over 70 kilometres off of Belize’s coast.

That means any significant medical emergency is immediately more dangerous by default because you’re looking at a long, bumpy ride back to the nearest hospital. That’s why most amateur divers without experience and training are not permitted to dive recreationally there. It’s just too dangerous with the distance from shore.

A key element of the Great Blue Hole’s danger is its sheer depth. As the deepest underwater sinkhole in the world, there are several factors that could diving here fatal. The first is buoyancy.

Now, humans float in water, we all know this; and most of you have probably doggy paddled in your local pool once or twice to prove it. The oxygen in our lungs, coupled with the natural buoyancy of a person, causes us to float on the water’s surface. But underwater, it’s a different story.

The deeper underwater you swim, the less you float. This is because the pressure of the water above you counteracts your body’s buoyancy. There is a point underwater where a person will neither sink or float, but go much deeper and a person’s body begins to have negative buoyancy, meaning that even though they are in water, they will start to sink.

Gives a whole new meaning to the word “sinkhole” now, doesn’t it? You can imagine an unsuspecting or relatively inexperienced diver going just a little too deep and without some powerful swimming, it’s too late. The scary part is, is the deeper you go the faster and faster you sink.

It’s 125 metres to the bottom. (And you thought asking your crush out was high pressure.)

Then you have the issue of nitrogen narcosis. The oxygen tanks used by divers confusingly do not actually contain that much oxygen. Instead, they are approximately 80% nitrogen, similar to the makeup of the air we breathe on the surface, makes sense right? At greater depths however, the amount of nitrogen we breathe can lead to a phenomenon known as nitrogen narcosis.

Many compare it to a somewhat buzzed or drunken state that you might experience through drinking alcohol where your judgment is impaired. At depth, you can see how this could be highly dangerous. Divers the world over tell stories of friends who start laughing to themselves before attempting to take off their breathing equipment. If they’re lucky, an eagle-eyed instructor will bring them back to a safer depth where it fades.

If they’re not, it can lead to mistakes that could cost a person their life.

Then you have the bends. Come up too quickly from the depths and the nitrogen bubbles in your blood can cause decompression sickness, which can be highly dangerous. This is why divers must ascend slowly.

So, the Great Blue Hole creates conditions where divers cannot go too deep or shallow at any one time. This requires a considerable level of concentration which could be compromised by nitrogen narcosis. Derek Low, a travel blogger, reported his experience of the depths playing tricks on your brain at the Great Blue Hole, quoting here: “And then things started going weird for me too.

I started experiencing vertigo, losing sense of what’s up and what’s down. I looked at my dive buddy. He looked 3 metres tall, like a giant.

I looked around, and I realized we were the only ones left at 45 metres. The divemaster was frantically signalling for us to ascend.” This hallucination in size could be what affected the divers who saw the “sea serpent” decades before.

Of course, you have the marine life also. Most sharks that travel through the sinkhole are just curious and will even come within arms’ length of some divers to inspect… whatever weird species of fish they think a human in a diving suit is. But that doesn’t mean there are no risks, they are sharks after all. Cut yourself here on sharp corals near the and you might find yourself leaving with less of your body than you entered with.

Overall though the marine life, like all wildlife should be treated with respect and it will in all likelihood leave you alone.

A lot of these dangers would probably be no issues for the more experienced divers. So, what about the highly experienced specialist and even professional divers? What dangers do they face? 45 metres is the limit for recreational divers, but beyond that is even more peril.

As you descend further into the sinkhole, between the stalagmites and stalactites there are various underwater tunnels and caves. Personally, I’d argue that caves are not fun places to be at the best of times, and as anyone with fear of the depths will tell you, infinite bodies of water that disappear into the darkness aren’t fun either. But you know what’s worse?

Putting those things together. Tight spaces, no oxygen, huge pressure and total darkness and that’s just the start. There are also silt deposits deep in these areas that, when disturbed, is kicked up into huge clouds that would disorientate any diver, professional or otherwise.

It could be very easy to get turned around in a cloud of silt and not even know it. And when everything looks the same underwater, an exit could easily be a dead end. In Richard Branson’s 2018 expedition to attempt to map the Great Blue Hole and its associated tunnels and caves, whole carcasses of sea turtles were found amongst some of them.

If even sea turtles, adapted to the water are unable to escape this kind of environment, what are the risks for a human whose body is entirely unaccustomed to it?

Going even deeper still you eventually reach a deep, dark void in the Great Blue Hole. This is known as the anoxic layer. Owing to the extremely low levels of oxygenated water this far down, a zone is created where there is almost no life at all, and nothing other than anaerobic bacteria can live. These bacteria eat sulphate in the water and produce hydrogen sulphide gas as a byproduct, which is toxic to humans and animals alike.

Erika Bergman, who was part of the 2018 expedition, states just what happens to critters who accidentally find themselves here, quote; “we found conches and conch shells and hermit crabs that had fallen into the hole and suffocated.” Yes, this is a place where sea creatures suffocate whilst underwater.

In summary, the deeper you go in the Great Blue Hole, you’re more likely to sink and drown, more likely to face cognitive impairment from the gases you breathe, more likely to get lost in a cave or the deep darkness below and more likely to swim through a layer of toxic gas that smells like rotten eggs. And if you manage all that, a shark might eat you anyway. The ocean is truly a horrible place. So, when you’re next spooked when you can’t see the other end of the swimming pool while under the surface, remember, it could always be much, much worse.

It’s important to note that many of these things happen in many deep-water dives around the world, not just here. In that regard, the Great Blue Hole is not much more dangerous or special than anywhere else. But it is also a tourist attraction bringing many people to these unique depths year-round with varying degrees of experience, so the chances of something going wrong here is higher than most.

But how many people have really lost their lives here? How many people, if this place is so dangerous, have really slipped below the surface never to return? Well, for all of the dangers of the Great Blue Hole, that’s actually quite a difficult thing to nail down.

There are no sets of statistics reported for those who do suffer injury or death at the Great Blue Hole so exact numbers are impossible to find. Some unconfirmed estimations based on local rumours have said that over 100 people have died in the Great Blue Hole. This wouldn’t be necessarily out of the question as another large sinkhole for divers in the Red Sea (creatively named “the Blue Hole”, evidently, it’s not so great) saw 130 people perish there between 1997 and 2012.

However, there is reason to believe that the number of people who have died at the Great Blue Hole off the coast of Belize is significantly fewer than that.

The 2018 expedition to map out as much of the sinkhole as possible did discover human remains, but not all that many and certainly not over 100 bodies’ worth. Erika Bergman who was on the trip, stated the following when they unfortunately came across the bodies of several divers, quoting here; “We found the resting place of a couple of folks, and we very respectfully let the Belize government know where we found them. Everyone decided that we would just not attempt any recovery.

It’s very dark and peaceful down there, just kind of let them stay.” A couple of divers is a far cry from the over 100 number other reports have suggested in the past.

This may well not be all of the bodies in the sinkhole, however, and it’s reasonable to suggest others may have been simply not found, either in the vast network of tunnels and caves or buried under the silt. However, it is unlikely to be many more if there are any more. Remains do not break down as quickly in the cold deep water, especially with far fewer bacteria in the anoxic layer and fewer predators that would dive deep to scavenge.

For all intents and purposes, whilst the danger of the Great Blue Hole is very apparent, the number of deaths appear to have been somewhat overblown. New evidence may one day come to light debunking this theory, but for now these are the facts we have. It is a stark reminder that even though diving can be fun and an exhilarating experience, that the deep ocean is something to be respected first and foremost.

Overall, the Great Blue Hole is a fascinating, mysterious and yes – deadly – place. It has captured the imagination of divers throughout the world over the decades. From mythical sea monsters to earthquakes, to billionaires, to skydiving, sharks, stalactites and a deeply unsettling lack of oxygen; there is something to fascinate the mind of almost anybody who comes across it. For a great big hole in the ground full of water, that’s pretty impressive.

I guess you could say it has a lot of depth.

Frequently Asked Questions

What is the Great Blue Hole?

The Great Blue Hole is a massive underwater sinkhole located in the Caribbean Sea, approximately 70 kilometres off the coast of Belize. It is the largest of its kind, with a diameter of over 300 metres and a depth of 125 metres.

Why is the Great Blue Hole popular among divers?

The Great Blue Hole is popular among divers due to its unique geological features, such as huge stalagmites and stalactites, and its clear waters with minimal currents. It offers both recreational and deep diving opportunities.

What are the dangers of diving in the Great Blue Hole?

Dangers include the risk of sinking due to negative buoyancy, nitrogen narcosis, decompression sickness, getting lost in underwater tunnels, and encountering toxic gases in the anoxic layer. Additionally, the remote location makes medical emergencies more dangerous.

How deep can recreational divers go in the Great Blue Hole?

The absolute depth limit for recreational divers in the Great Blue Hole is 45 metres. Going deeper has proven fatal even for experienced divers.

What marine life can be found in the Great Blue Hole?

Common marine life includes angelfish, butterflyfish, small groupers, and species of sharks and rays in the shallower areas. Coral and marine life can also be found on the walls of the sinkhole before it descends.

What is the anoxic layer in the Great Blue Hole?

The anoxic layer is a deep, dark void in the Great Blue Hole where there is almost no life due to extremely low levels of oxygenated water. Anaerobic bacteria produce hydrogen sulphide gas, which is toxic to humans and animals.

How many people have died in the Great Blue Hole?

Exact numbers are difficult to determine, but unconfirmed estimations suggest over 100 deaths. However, the 2018 expedition found only a few human remains, suggesting the number of deaths may be significantly fewer.

What is the history of the Great Blue Hole?

The Great Blue Hole was originally formed as a limestone cavern during the last ice age. An ancient earthquake caused the ceiling to collapse, creating the sinkhole structure. Sea levels rose and filled it as the ice age melted.

Who made the Great Blue Hole famous?

Jacques Cousteau made the Great Blue Hole famous in 1971 when he explored it using his mini submarine. He confirmed that the sinkhole had formed from a limestone cavern.

What did the 2018 expedition to the Great Blue Hole discover?

The 2018 expedition, funded by Richard Branson, mapped the sinkhole and found an empty two-litre bottle of Coca Cola, a lost GoPro camera, and the remains of a few divers. They also discovered human remains and sea turtle carcasses in underwater tunnels.

Sources

• Original Places video: The Great Blue Sinkhole: A Diver’s Worst Nightmare
• Hero image source by Chairman of the Joint Chiefs of Staff / openverse, by.