Belize's 120m-deep Great Blue Hole was formed 10000 years ago. Photo / Getty Images

The celebration of the first divers to survey the bottom of Belize's giant sinkhole was tempered by the discovery that plastic pollution had beaten them to it.

The Great Blue Hole draws the viewer and tourists to it like some like some tropical singularity into which the ocean drops away. It's one of the Central American country's most iconic landmarks.

However at the bottom of its pristine natural beauty divers have found a dirty secret.

The perfect circular pool descends 120 metres, creating a dark ring of water. It is at this depth that the team led by Virgin founder Richard Branson made the shocking discovery of plastic bottles and pollution.

Even here - at this place never before visited by humans – the ocean floor cannot escape the impact of man-made pollution.

At this popular tourist site, the expedition of two three-person submarines were deployed to explore the depths. The aim of the mission was to measure the extent of climate change on the ocean cave system, though they had not been expecting to discover plastic pollution at such extreme depths.

Among the team making the dive was businessman Richard Branson and Fabien Cousteau, the grandson of celebrated marine biologist Jacques Cousteau.

The depths were so hostile that even deep sea crustaceans could not survive the pressure and oxygen starvation.

As Branson noted on arriving at the bottom of the sink, "we could see crabs, conchs and other creatures that had fallen into the hole, arrived on the bottom and then ran out of oxygen and died".

An eerie wasteland, the real mission was to document the changes to sea levels in the rock walls – formed 10,000 years ago the sinkhole has a perfect record of the heights of the oceans and changes to climate spanning thousands of years.

In an entry on the expedition website Branson wrote of the importance of the hole to the mythology of the Mesoamerican people – jokingly saying the expedition was hoping to find some of these Mayan monsters.

However, it was the levels of pollution his team discovered in the untouched ocean chasm that really frightened the millionaire.

"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," he urged on the expedition page.

The journey was in part sponsored by Branson's new venture, the cruise line Virgin Voyages.

The business tycoon took the opportunity to pledge that his liners will not carry disposable plastic.

When Voyages' first ship Scarlet Lady sails on her maiden voyage in 2020, it will be stocked only with reusable and recyclable materials.

"There will be no single-use plastic on-board."

The move comes after the largest cruise line Royal Caribbean International made a pledge to rid throwaway plastics from its fleet in February 2018.

However, although cruise ships may be the closest source of waste to the oceans' natural wonders, they only make a small proportion of the waste that ends up in our waters.

Cousteau who joined the expedition also had a strong familial connection to the sink hole.

In 1971, Jacques Cousteau, Fabien's grandfather, popularised the world's largest sinkhole by leading his team of divers to explore the abyss.

It is Fabien's hope that the expedition can work to re-energise public interest in the site and use the attention to steer the conversation on the impact of climate change on our oceans.

The documentary filmmaker and conservationist joined the expedition to the site with the aim of promoting a campaign to conserve 30 per cent of the world's oceans as protected areas by 2030.

For now the Blue Hole in Belize is one of seven formally recognised areas of protection in the Mesoamerican reef, but the discovery of what lies beneath shows there is much further to go in protecting this fragile ocean environment.