Richard Branson was expecting his recent journey into the world's largest sinkhole to uncover "Mayan mysteries and myths of monsters and wonder".

Instead, the billionaire Virgin founder discovered plastic bottles and signs of climate change.

Branson and ocean conservationist Fabien Cousteau completed the first ever submersible dive to the bottom of Belize's Blue Hole, a sinkhole big enough to swallow two Boeing 747s with room to spare, last month, broadcasting the mission on the Discovery Channel.

"As for the mythical monsters of the deep?" Branson asked in a recent blog post. "Well the real monsters facing the ocean are climate change - and plastic.

FLIGHT CENTRE The Blue Hole lies 64km off the coast of Belize.

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"Sadly, we saw plastic bottles at the bottom of the hole, which is a real scourge of the ocean. We've got to get rid of single-use plastic."

Located in Lighthouse Reef off the coast of Belize, the 124-metre deep, 318m-wide sinkhole is part of the UNESCO-listed Belize Barrier Reef System and a magnet for scuba divers.

About 300 feet (91 metres) down, Branson, Cousteau and Aquatica Submarines' chief pilot Erika Bergman came across a wall of stalactites - evidence, Branson says, of a cave system which would have originally formed on dry land.

"It is proof of how oceans can rise quickly and catastrophically. Sea levels were once hundreds of feet lower... 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."

The team also found skeletons of crabs, conches and other sea creatures at the bottom, which had run out of oxygen after descending into the hole's hydrogen sulfide layer.

Sponsored by Virgin Voyages, the team hoped to gather information on water quality and oxygen levels, and record high-resolution footage of the sinkhole's interior.

They also wanted to help Ocean Unite promote ocean awareness and achieve its goal of having at least 30 per cent of the world's ocean protected by 2030.

Branson said the team spoke with Belize Prime Minister Dean Barrow about protecting 10 per cent of the country's waters and banning grill nets, "which are known as the Wall of Death and do so much damage to marine ecosystems".

AQAUTICA SUBMARINES Descending into the Blue Hole.

"My grandchildren will be in their thirties by 2050," Branson said. "I don't want them to grow up in a world without corals, without the wonders of the ocean. We need governments to act now to protect at least 30 per cent of the ocean by 2030 and reduce CO2 emissions as quickly as possible with a goal of zero net emissions by 2050."