A recent expedition has taken the plunge into the Great Blue Hole, the curiously alluring marine sinkhole inside the Lighthouse reef of Belize, and brought back a collection of stunning photographs detailing this geological oddity.

Between November 27 and December 13, 2018, an expedition team conducted over 20 dives into the Blue Hole using Aquatica’s Stingray submarine and the Roatan Institute of Deepsea Exploration’s (R.I.D.E.) Idabel submarine. The crew includes none other than Richard Branson, famously rich human and founder of the Virgin Group, and Fabien Cousteau, grandson of famous underwater explorer Jacques Cousteau who brought attention to the Blue Hole in the early 1970s, along with a host of scientists and submarine experts.

Along with some impressive holiday snaps, their work also produced a 3D sonar map of the Blue Hole and gathered environmental data about its water. Once processed, all of this information will be shared with the Belize government and the global scientific community to help preserve the Great Blue Hole and its biodiversity.

At over 300 meters (984 feet) wide and 125 meters (410 feet) deep, the Great Blue Hole is the second largest marine sinkhole in the world, following the Dragon Hole in the South China Sea. It’s found in the Belize Barrier Reef Reserve System, a UNESCO World Heritage site. Charles Darwin described this reef as “the most remarkable reef in the West Indies.” And for a good reason too: the area is home to a number of unique reef types and is among the most pristine reef ecosystems in the Western Hemisphere. It's also awash with biodiversity, regularly visited by an array of marine creatures, including hammerhead sharks and reef sharks.

"Over the past 14,000 years the polar ice caps, formed during the last glacial maximum, have thawed and raised sea level in steps. These defrosting events are captured in a stone record of an oceanic sinkhole in Belize," Erika Bergman, Chief Pilot and Director of Operations of Aquatica, wrote in a blog post.

"The aptly named Great Blue Hole is a collapsed cave, filled with stalactite caverns, and built up from layers of fine limestone and rougher calcium carbonate walls," she added. "Preserved from the disturbance of time, and isolated in the darkness, the hole holds clues to a very natural part of our planet’s life cycle. It’s these terraces and stalactites we set out to map." 

One of the Great Blue Hole’s most interesting features is its hydrogen sulfide layer. At a depth of around 90 meters (~300 feet), a cloak of hydrogen sulfide is found within the hole. This stuff is toxic, corrosive, and it really, really stinks. As you sink a little deeper towards the bottom, the water also becomes absent of oxygen (anoxic). This results in a graveyard of shellfish at around 106 meters (~350 feet) where thousands of unsuspecting creatures have dared to swim too deep and perished.