Explorers made hauntingly bleak discovery after finally reaching bottom of Great Blue Hole

Richard Branson's expedition to the Great Blue Hole proved both revelatory and harrowing.

The 400-feet deep marine sinkhole was immortalised by oceanographer Jacques Cousteau after he visited the site in his ship, the Calypso, 54 years ago, and declared it to be one of the top five scuba sites on Earth.

Together with Aquatica Submarines, billionaire Branson, Cousteau's grandson Fabien and National Geographic's Erik Bergman eventually got a chance to see it for themselves in December 2018.

However, what they found at the bottom of the Great Blue Hole was the stuff of nightmares and a stain on human society.

The Great Blue Hole (YouTube/Will Mitchell)

Where is the Great Blue Hole and how deep is it?

The Great Blue Hole is located just off the coast of Central American country Belize.

Measuring 400ft in depth and approximately 1,050ft in width, the UNESCO World Heritage Site is the largest oceanic sinkhole in the world.

The mission to reach the bottom of the blue

This mission was to dive down to the bottom of the sinkhole and explore its underwater cave system. Two three-person submersibles took the plunge, according to the Daily Mail, with divers, geologists, and other researchers conducting companion research projects, including a 3D scan of the area.

A haunting discovery and the difficult decision that was made

The submarine expedition encountered the bodies of two divers at the bottom - three were believed to have gone missing long ago - and reported them to Belizean authorities.

"We found kind of the resting place of a couple folks, and we just sort of very respectfully let the Belize government know where we found them, and everyone decided that we would just not attempt any recovery," Bergman revealed to Business Insider.

"It's very dark and peaceful down there, just kind of let them stay."

It is located some 60 miles off the coast of Belize (Elliott Pelling/Getty Images)

A place where no life can survive

The explorers found that at a depth of 290ft, there is a layer of hydrogen sulfide; a colourless, flammable, and toxic gas, which no marine life can safely penetrate.

"Anything that fell into the hole decomposed until that process used up all of the oxygen below 290ft," Bergman told Forbes in 2019, adding that there's no oxygen beneath this killer layer.

"Without oxygen, nothing survives. Beneath the hydrogen sulfide layer it is very dark. Looking straight up from inside the submarine you can just barely make out the circular opening of the hole, all of the light pouring into the hole is shut out by the thick chemical layer."

The disturbing discovery that should be a warning to the world

The bottom of the seafloor was also covered with dead sea snails (conches to be exact) and shockingly, plastic bottles.

David Attenborough, who turned 99 last week, would be especially appalled to hear that.

Branson would later write on his personal blog: "The real monsters facing the ocean are climate change – and plastic. Sadly, we saw plastic bottles at the bottom of the hole, which is a real scourge of the ocean. We’ve all got to get rid of single-use plastic."

Meanwhile, Bergman explained of the burying process down in the depths: "Some of the targets we were particularly interested in studying in detail were the stalactite caverns from 130 feet to 160 feet deep, formed back when sea level was 500 feet lower, and this was a dry cave. We also observed the calcium carbonate layer at 290 feet where a great coral reef grew in what was then shallow Caribbean waters.

"Among the rocky outcrops and fallen ledges plunged in the silt around the circumference of the Hole, there was also a concentric circle of sand. Once something falls in the Hole it can’t be washed away in deep sea currents, so anything down there stays put as waterfalls of sand slowly fill in the Hole."