IIt looks like, at first, like a giant, flat, ink blot in the sea, but underneath this sinkhole – the biggest in the world – is a cavern large enough to swallow two Boeing 747s with room to spare.
Famed marine explorer Jacques Cousteau did not discover the sinkhole, located in the Caribbean off the coast of Belize, but he called it "The Great Blue Hole" in 1971, and it's been a magnet for scuba divers ever since.
Now, in the first mission of its kind, Cousteau's grandson and Sir Richard Branson are plunging into the deep, dark bottom of it in a submarine as part of an expedition that will be streamed live and broadcasted globally on the Discovery Channel.
This will be quite a feat. Scuba divers generally only descend to a maximum of 130 feet underwater, so what lies beneath that remains largely uncharted territory, and there's a lot of it.
Branson and Fabien Cousteau will join Aquatica Submarines' chief rider Erika Bergman and make several expeditions into the sinkhole this weekend in a remote-piloted Stingray 500 submarine, to collect data and map out the submerged cave.
Some 100,000 years ago, this natural marvel was a network of caves that sat above the level, with ceilings hanging with limestone stalagmites and stalactites. Over time and during our planet's latest glacial period, rising sea levels eventually flooded the structure and collapsed to form the sinkhole it is today.
The Great Blue Hole was measured by scientists using sonar technology in 1997, but this will be the first since then and by far the most thorough exploration.
Bergman's team hopes to gather scientific data on marine aspects including water quality and bacterial activity, as well as achieve high-resolution footage and a detailed plan for the Hole's internal structure for the first time.
Intriguingly, they are attempting to discover what is theorized to be an oxygen-depleted layer at its base which could provide vital clues about environmental forces potentially related to the fall of the civilization Mayan between 800 and 1000 AD.
"One of the most interesting marks that we are really excited to do is oxygen testing," Bergman told Endgadget. "We've heard that in the Blue Hole there is an anoxic layer near the bottom [and] things do not degrade in anoxic areas so we could find preserved life. "
Branson is hoping that its involvement in the project will generate awareness around ocean conservation, and is supporting the goal of protecting at least 30 percent of the ocean by 2030.
The broadcast will be live on the Discovery Channel from 9-11pm on Sunday, December 2, and Richard Branson will be speaking to Telegraph Travel about what the team will unleash next week.