Exploring the Great Blue Hole

The largest marine sinkhole in the world has been explored and mapped by a group of aquanauts including Sir Richard Branson and Fabien Cousteau.

The Great Blue Hole is the largest marine sinkhole in the world

Over 40 miles off the coast of Belize lies one of the most bizarre and intriguing oceanographic features in the world, the Great Blue Hole. Measuring 1,043ft (318m) across and 407ft (124m) deep it is the largest marine sinkhole on the planet. It is a popular dive site and has inspired many myths and legends due to its strange circular shape and astounding depth compared to surrounding waters. It was made famous when in 1970 iconic French explorer Jacque Cousteau not only named it but became the first person to reach the bottom in an early submersible devise. Now almost 50 years later a team including his grandson Fabien and British philanthropist Sir Richard Branson have repeated the accomplishment. The dive was to properly explore and map the sinkhole for the first time as well as being part of a live TV programme on the Discovery channel.

What is a marine sinkhole?

A sinkhole is basically just a collapsed cave system that has an open top. Most cave systems are formed underground when acidic water dissolves the limestone in the earth creating hollowed out spaces. When the cave gets too large and the roof gets too thin it can collapse. This leaves a natural hole in the ground which can range from the size of a small car to hundreds of metres across. Marine sinkholes are versions of these that have been filled with seawater over time due to climatic change and sea level rises. The Great Blue Hole is a sinkhole that formed above sea level and was then flooded around 20,000 years ago. It is also the largest of its kind and is deep enough to stand the statue of liberty inside it with a hundred feet left to spare.

The mission

A three person team were sent down in the submersible devise known as Stingray 500, which was provided by Aquatica Submarines who also took care of planning the dive. The first member was billionaire philanthropist Sir Richard Branson who has funded multiple ocean exploration challenges and described it as an ‘awesome’ experience. When asked why he felt it was important he replied “the fascinating thing about going down underneath the ocean is that less than 5% has been explored”. He was joined by Fabien Cousteau the grandson of the famous Jacque Cousteau who first reached the bottom of the Great Blue Hole and made it famous.  He claimed the experience was “emotionally charged following in his grandfather’s finsteps” (yes he really said finsteps) and praised the technology that allowed them to pull off the endeavour. The team was rounded off with Aquatica’s chief pilot Erika Bergman, the youngest ever female sub pilot, who guided her high profile passengers all the way to the bottom and safely back again.

A shot of the crew from Blue Hole: Live (Left to Right: Fabien Cousteau, Erika Bergman and Sir Richard Branson)

The aim of the dive was to, for the first time in almost 50 years, reach the bottom of the sinkhole whilst recording things about it that were previously unknown. Another reason behind the endeavour was to stream the dive live to the world via the Discovery channel in a programme titled Blue Hole: Live. This was to inspire people about ocean exploration just as Jacque Cousteau had in 1970 when he also recorded parts of his experience. However due to poor weather conditions predicted for the day the programme was scheduled, including 30 knot winds and large swells, the dive could not go ahead. Luckily the day before a break in conditions and provided the team with their chance and they took it, diving on 2nd December. The TV programme the next day then consisted a live playback of the dive as well as interviews with the aquanauts and supplementary informational videos. This was not the ideal scenario but considering the conditions it was lucky they were able to dive at all.

The subs

Two Aquatica subs were actually used for the dive, firstly the Stingray 500 and secondly the IDABEL which was used primarily to light the way and help film the event. The Stingray 500 is described by pilot Erika Bergman as “the best sub I’ve ever piloted” and is a state of the art vehicle. Capable of going to a depth of 500ft (as the name suggests) it is equipped with HD cameras, SONAR and an acrylic dome giving its occupants a 270 degree view. This meant that the crew would have tools capable of allowing them the best view yet of the marine sinkhole and the capacity to document it like never before. Compared to Cousteau’s 1970 ‘saucer’ which only allowed for a few minutes at the bottom before having to resurface the Stingray allows its inhabitants to remain down for hours. In fact it has the capacity to maintain life support systems for 96 hours should something go really wrong.

Aquatica’s Stingray 500 submersible devise

Although the sub is an impressive piece of technology that was vital to the dive it is nothing without a competent pilot. The people controlling them like Erika have got to be extremely aware of their surroundings including currents, wildlife and rocky terrain. As well as this they must also maintain optimal life support for their passengers and remain in contact with the surface. Due to the fact Scuba divers are limited to 170ft everything below that depth was pretty much unknown meaning that the dive required ‘months of meticulous planning’.

The dive

As the three aquanauts began their journey floating on the surface Cousteau remarked to Branson, owner of Virgin Galactic, that “it’s a little slower than a rocket launch, but just as exciting nonetheless”. But soon enough they were completely submerged and began their decent into the blackness below. The first twenty metres were what you’d expect in Belize’s tropical waters, crystal clear visibility and near pristine coral reefs around the edge of the hole. But after descending below the lip it began getting much darker and far more murky and lifeless. At around 40m the team came across the sinkholes infamous stalactites and stalagmites, up to 12m in length. They mark the entrance to several underwater caverns which are frequented by experienced divers, the crew even claimed they could see some trapped air bubbles from their Scuba systems. As the formations can only be made out of water they are proof that the sinkhole used to be an above sea cave system.


As they continued down they came across several ‘terraces’ which are defined lines of geological distinction in the rocks that provide further evidence of major sea-level changes over time. At this point in their dive everything they were looking at had never been observed by humans before them. What happened next at around 90-100m in almost complete darkness was particularly interesting. The team came across a hydrogen sulphide layer which, due to its density relative to water, created a cloud that the sub could ‘sit’ on without descending. Below this layer the water was anoxic meaning that they encountered several well preserved remains of animals that had fallen into the sinkhole and died before they could escape.

The team resurfaces as the sun begins to set

When the team finally reached the bottom they found it to be exceptionally silty and littered with rocky debris. This is unsurprising as the sinkhole acts as a sort of drain for the surrounding waters allowing sand and organic matter to drop to the deepest point. The rocky debris was the remains of the cave roof that had become too thin and collapsed when it was still above water. After staying for a while they began their accent back up to the surface. Due to the fact the pressure remained constant in the sub during the entire dive this was relatively straight forward as they did not have to worry about decompression. In total the dive took over 2 hours, which was shorter than initially planned due to the weather, but was an overwhelming success considering the challenges they faced.

What next?

The Aquatica team will remain in the area surrounding the Great Blue Hole for two weeks and continue to survey the site by launching multiple other dives. Using the subs SONAR capacity they aim to create a highly detailed 3D map of the Great Blue Hole as well as survey marine life. It is hoped that studying the sinkhole could provide further evidence of how sea-levels have changed naturally over time and can provide clues as to how they are likely to change in the future. It is important that ocean exploration like this continues into the future as we know so little about a vast majority of the world’s oceans and the effects we might be having on them.





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