Take a glance at the Great Blue Hole, the world-famous marine sinkhole in Belize, and you'll no doubt wonder what's at the bottom of that thing? Well, prepare to lose a little bit of faith in humanity. 

Richard Branson, famously rich human and founder of the Virgin Group, recently headed to Central America to explore this fascinating blob of blue using a submersible alongside a team of scientists and filmmakers from Discovery. Writing in a blog post, Branson explained the venture was the "first ever submersible dive to the very bottom of the Blue Hole on the Mesoamerican Reef, the second largest barrier reef network in the world." 

Sadly, even this remote blue abyss – some 100 kilometers (60 miles) off the Belize coast – is not safe from the affliction of plastic pollution.

“As for the mythical monsters of the deep? Well, the real monsters facing the ocean are climate change – and plastic,” said Branson. “Sadly, we saw plastic bottles at the bottom of the hole, which is a real scourge of the ocean.“

Other than a few bits of plastic trash, Branson and the team also observed a collection of dead crabs and other creatures that had descended beneath the hole's hydrogen sulfide layer, where there's little to no oxygen.

As you can imagine, the Great Blue Hole is a major pull for tourists. Its crystal clear waters and rich marine life is a hotspot for scuba divers and boat tours, so the presence of plastic trash is perhaps unsurprising. After all, researchers discovered plastic bags at depths of 10,898 meters (35,755 feet, or 6.7 miles) on the floor of the Mariana Trench, the world’s deepest ocean trench.

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 followed by the Dragon Hole in the South China Sea. These remarkable natural features are actually a product of past ice ages. Like most other marine sinkholes, it was once a system of limestone caves with a carbonate bed as a roof. Then, at the end of the last glacial period, sea levels rose by hundreds of feet, collapsing the bed and flooding the cave with water. As a subtle reminder of their ancient history, you can still see stalactites lining the inside walls of the hole, just like the ones you would see in a cave.

“Hopefully by this trip taking place we have raised even more awareness of the need to protect the ocean and tackle climate change now – before it is too late,” Branson said.