Formation and Geology of the Great Blue Hole, Belize, Caribbean Sea

An amazing geological feature lurks under the Caribbean Sea. Famously seen from above and explored from below, this is the Great Blue Hole. I was recently in Belize and got to sail to and dive within the notorious Great Blue Hole. It is an ominous underwater sinkhole that invites the curious explorer to get close… but not too close. In this post I will talk about the geology and environmental factors about how it formed, as well as my personal experience sailing to and scuba diving within the Great Blue Hole in the Caribbean Sea.

Intro to the Caribbean Sea

The Caribbean Sea is a famous sea surrounded by many tropical island countries. It also happens to be famous for its pirates, both in the past and popularized in modern cinema. However, one of the most remarkable features of the Caribbean Sea is its barrier reef. The Caribbean Sea hosts the 2^nd largest barrier reef (coral reef) in the world, second only to the Great Barrier Reef of Australia. This is called the Mesoamerican Barrier Reef System, also known as the Great Maya Reef. A fundamental part of the Mesoamerican Barrier Reef is the Belize Barrier Reef System.

The Belize Barrier Reef System contains spectacular coral structures and abundant sea life. Within these relatively shallow waters lies the famous Great Blue Hole. The Great Blue Hole is nested within Lighthouse Reef atoll, about 80 km from mainland Belize and part of the Belize Barrier Reef system.

Lighthouse Reef Atoll Wonders

Lighthouse Reef Atoll stands out on its own as a stunning mix of turquoise waters, coral reefs and small cayes, such as Half Moon Caye, located at the southeast corner of the atoll. This, like the Great Blue Hole, is a Natural Monument and protected area. Rare bird species, like the red-footed boobie (in the rare white phase), cousin of the Galapagos’ famous blue-footed boobie, nest there. Half Moon Caye also contains littoral forests and “paradise island” coconut palm beaches, with white sand welcoming to nesting turtles and many hermit crabs. Here you can purchase the park pass for visiting the island and visiting the Great Blue Hole.

The Great Blue Hole is located in the centre of Lighthouse Reef Atoll and stands out as a dark blue, ominous circular feature within the brilliant turquoise and aquamarine shallow waters. It is best seen from afar and even strikingly visible from space. It is roughly 1000 feet (305 m) in diametre and 412 feet (125 m) deep. It is a UNESCO world heritage site and frequently listed amongst the top wonders of the world. It was also made famous by the visit of Jacques Cousteau’s RV Calypso in 1970, which boosted its popularity as a top dive site in the world. Though unfortunately, its intriguing nature has taken the lives of many divers. This is because below a depth greater than 100 feet (30 metre) nitrogen narcosis (or gas narcosis) can occur, and the effects become greater with greater depth, like 130 feet and below. It is caused by breathing the compressed gas at depth, and can cloud the reasoning and judgement of the diver, such as feeling like you are intoxicated or overconfident or euphoric. Thus, divers can keep going deeper and deeper, beyond the recommend safety 130 feet part, and don’t come up…

Geology of the Great Blue Hole

The Great Blue Hole is a Pleistocene-aged, drowned offshore karst feature, which in other words, is an underwater sinkhole that formed > 10,000 years ago.

The rim of the Great Blue Hole is thriving with marine sea life such as corals and colorful fishes. This shallow rim of ~ 5 m depth gradually slopes to 10 m and into a coarse sediment slope, until dramatically reaching a vertical cliff that encircles and defines that Great Blue Hole. Due to the poor circulation of oxygen and lack of light at the bottom of the hole, the environment quickly changes to inhospitable. The small, vibrant fishes are replaced with large, lurking sharks. Specifically, below 90 m depth the water in the Blue Hole is anoxic (Dill, 1977).

The Great Blue Hole originally formed above water, as a cave. Cave systems form due to slightly acidic waters peculating through the reactive limestone rocks. This slowly dissolves parts of the rocks, leaving large holes and cavities, and also creates features like stalactites (top-down) and stalagmites (bottom-up) via slow percolating waters depositing calcium dissolved in the water (famous seen in other cave systems). The limestone rocks of the cave formed during the last ice age and are as old as 153,000 years, but had a variable historic of formation with events at 65,000 years ago and 15,000 years ago (Gischler et al., 2013). The cave system formed during the latter bits of this time frame, when sea level was much lower than today and the cave system of the Great Blue Hole was exposed above water.

Since the end of the last ice age (~10,000 years ago), sea levels have gradually risen and the above-water cave system that was the proto-Great Blue Hole, slowly got flooded and submerged. The cylindrical shape of the Great Blue Hole resulted from the eventual collapse of the roof of the karst cave due to instability, which probably collapsed earlier during the Pleistocene (Gischler et al.,, 2017). Evidence of this is found at the bottom of the hole, with fragments of the roof collapse (Gischler et al., 2008).

Another neat scientific aspect of the Great Blue Hole is that it has helped act as a record of the climate in the Caribbean. Due to its isolation, it has had a relatively undisturbed, annual layer of sediments on its bottom. This has aided scientists to serve as a high-resolution record of climate and storm archives over the last 1000s of years (Gischler et al., 2013).

Dive Experience

As a self-described “rusty” scuba diver that hasn’t been to significant depths before, I was quite nervous and aware of my inabilities. While I do have my scuba dive PADI certificate (from Australia), I haven’t frequently dived too much since, spending more time of course in the mountains looking at rocks as geologist, than in the waters. However, I found myself on a sailboat with my mother and aunt on an adventure, with a caption who was a professional scuba diver throughout his career. After several refresher dives prior to the Great Blue Hole, I felt ok about the attempt, knowing that I have good situational awareness, and was with a reliable scuba partner.

We sailed our boat through the challenging shallow waters towards the Great Blue Hole and hooked up on a mooring buoy. The decent was surprisingly easily, and we went down to 120 feet in no time. It was quite amazing to see the desolate landscape of the cave walls where few creatures lurked, expect for the sharks circling above. I could see how easily it can be to keep going deep without being aware of the danger… But luckily, we had full awareness and a strict 8 minute bottom-time plan before we then ascended and decompressed below the surface before breaching the top.

Final thoughts

The Great Blue Hole lives up to its name, albeit, surprisingly more ominous in person. Navigating on a sailboat to the Great Blue Hole in Belize was no small feat, with dangerously shallow waters and inconspicuous coral heads lurking at uncomfortable distances. Once at the Great Blue Hole, the deep blue hole is hard to make out at sea level, but with the aid of a drone you can see the true nature of the feature. Being able to submerge and dive into the geological wonder, seeing the vertical cliff drop and old stalagmites and cave formations is truly a striking experience, albeit quite scary and done with extreme respect and caution. And knowing more about the geological history and formation makes you appreciate the beauty and rarity of the site even more.

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