The Great Blue Hole is a 124 m deep sinkhole in the Caribbean off the coast of Belize – a marine counterpart of a Yucatán cenote but without the horizontal cave development. There is little or nothing alive at the bottom due to a sterilising layer of hydrogen sulphide at 91 m deep. But plenty of dead sea life including two human skeletons, thought to be of divers gone missing. Plus of course the inescapable plastic trash.

Richard Branson, who explored the Hole in 2018 in the company of oceanographic explorer Fabien Cousteau and National Geographic Explorer Erika Bergman, had this to say at the time:

At 300 feet [91 meters] down you could see the change in the rock where it used to be land and turned into sea. It was one of the starkest reminders of the danger of climate change I’ve ever seen.