Ever since famed ocean explorer Jacques Cousteau brought worldwide attention to Belize's spectacular Great Blue Hole in the early 1970s, fascination and curiosity have grown over what the bottom of this dark blue natural wonder looks like.

In December of last year, a team composed of billionaire Richard Branson, submersible pilot Erika Bergman and documentary filmmaker and ocean conservationist Fabien Cousteau became the first to find out –– plunging more than 400 feet to the bottom of the sinkhole. While Branson described the stark landscape that greeted them as "extremely eerie," it sadly was not completely devoid of the unfamiliar.

"As for the mythical monsters of the deep? Well, the real monsters facing the ocean are climate change — and plastic," he wrote in a blog post. "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."

In another dive, Bergman reported that the team also recovered a lost GoPro with an intact SD card. "One less piece of plastic ..." she wrote on Branson's Instagram account.

In addition to exploring its depths, the month-long expedition also completed a first-ever interactive 3-D scan of the site.

"It is a virtual map, and that data will be provided to the Belize government for research purposes, so they can understand more about the Blue Hole and help contribute to its conservation," Bryan Price, vice president of Aquatica Submarines, told The San Pedro Sun. "We are doing a bathymetric survey with another partner, and we are also going to be doing some observational science, so we will be embarking fisheries officials and other people like that, students, to go down and really observe things in the (Belize) Blue Hole that matter to them."

Monsters of the deep

This isn't the first time pioneers to the ocean's depths have been disappointed in this way. In 2017, researchers studying marine creatures captured at the bottom of the Mariana Trench — the ocean's deepest point at more than 36,000 feet — were shocked to discover that 100 percent of them were found to have ingested plastic.

"The results were both immediate and startling," said study lead Dr. Alan Jamieson of Newcastle University. "This type of work requires a great deal of contamination control, but there were instances where the fibers could actually be seen in the stomach contents as they were being removed."

In 2018, scientists studying video and photos captured from the bottom of the Mariana Trench found one containing a plastic bag. That's now considered the deepest known piece of plastic trash on Earth.

Scientists now believe the ocean's deep spots, known as the hadal zone, may be acting as repositories for large amounts of plastic pollution. Just last month, a study published by researchers from the Chinese Academy of Science found as many as 2,000 pieces of microplastic in a one liter sample of water taken from the Mariana Trench.

"Manmade plastics have contaminated the most remote and deepest places on the planet," the Chinese scientists wrote. "The hadal zone is likely one of the largest sinks for microplastic debris on Earth, with unknown but potentially damaging impacts on this fragile ecosystem."

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.

Plastic discovered at bottom of Great Blue Hole
Even the Great Blue Hole in Belize, one of the world's largest sinkholes, isn't immune to human pollution.