Man-made horrors lurk at the bottom of the Mariana Trench, the deepest part of the ocean

A helium balloon decorated with characters from the film Frozen at the bottom of the Mariana Trench. Photo / Mariana Trench - In Pursuit of the Abyss

The Mariana Trench is the deepest part of the ocean, and the deepest part of Earth itself, measuring some 11km down.

So unforgiving are the pitch-black, pressured and near-freezing conditions that we know little about what lurks below, with untold marine treasures still waiting to be discovered.

Scientists consider the absolute lowest beds of the sea to be about as hard to reach as space.

Victor Vescovo was the last human to do so, reaching a new record depth of 10.9km in April 2019 in a Triton 36000/2 submarine, built to withstand the extreme pressure.

Over five dives to the bottom of the Mariana Trench, the American explorer found previously unknown crustacean species, witnessed brightly coloured outcrops and came across a pink snailfish.

A scene from the film Mariana Trench - In Pursuit of the Abyss shows plastic waste found by unmanned craft. Photo / YouTube

Then, scattered throughout a place only two others have ever managed to physically reach, Vescovo saw plastic.

Lolly wrappers and a plastic shopping bag, to be precise.

Nowhere is safe from humanity

Vescovo's shock find almost overshadowed his remarkable achievement and the scientific promise of his sea life discoveries.

And for good reason.

"We always had this sense that there was a part of the planet that was beyond, that was untouched by human action," Eric Galbraith, an ocean biochemist at the Catalan Institution for Research and Advanced Studies in Barcelona and adjunct professor at McGill University in Montreal, told the magazine Maclean's.

"That used to be true. And now it's no longer true."

And unfortunately, Vescovo's discovery isn't the first, with previous unmanned voyages to the depths of the Mariana also encountering plenty of plastic pollution.

The Great Pacific Garbage Patch contains 1.8 trillion pieces of plastic waste. Photo / Phys.org

Showing how extensive the problem is – and how quickly waste can sink down – one dive found the remnants of a helium balloon decorated with characters from the children's film Frozen, released in 2013.

Vision captured of that dive shows the balloon and, resting next to it, a heavy-duty 20-litre plastic bucket.

In 2018, researchers from the Institute of Deep Sea Science and Engineering in China took samples of water and sediment at depths ranging from 2.5km to 11km.

"Man-made plastics have contaminated the most remote and deepest places on the planet," they wrote in analysis published in the journal Geochemical Perspectives Letters.

"The (deepest Mariana) zone is likely one of the largest sinks for microplastic debris on Earth, with unknown but potentially damaging impacts on this fragile ecosystem."

Will pollution wake-up call be heard?

Startlingly, the deeper the sample collected, the higher the concentration of microplastics. Within sediment samples taken at the bottom, they measured 2200 pieces per litre.

Much of the microplastic they discovered was in the form of fibres measuring a few millimetres long, likely to have come from polyester clothing, bottles and packaging.

And while it's small, it's far from harmless, the researchers pointed out. It harms sea life, which is already at significant risk from climate change, other pollutants and overfishing.

Encountering waste in such a remote part of the ocean is a wake-up call, scientists say.

But given much more jaw-dropping signs of humanity's destruction have been visible for decades, it's unlikely the warnings will be heeded.

Millions of tonnes of plastic makes its way into oceans each year and while half of it sinks, the rest is less dense than seawater and therefore floats.

A black footed albatross chick with plastics in its stomach lies dead on Midway Atoll in the northwestern Hawaiian Islands. Photo / US Fish and Wildlife Service

A huge shared impact

Given how interconnected the oceans are, plastics are moved great distances by tides, currents and gyres, whether it's bobbing on the surface or being sucked along beneath it.

Take the accidental release into the sea of 28,000 yellow rubber duckies in 1992.

A cargo ship carrying them hit a horrific storm and lost several containers during a trip from China to the United States.

Scientists eventually tracked the yellow ducks all over the world. They had reached every single part of the planet's seas.

The Great Pacific Garbage Patch is a 1.6 million square kilometre stretch concentration of plastic waste floating between California and Hawaii.

It's slightly larger than Australia's Northern Territory.

Experts estimate the Great Pacific Garbage Patch – the biggest of five accumulation zones in the world – contains 1.8 trillion pieces of plastic, weighing some 80,000 tonnes.

"When accounting for the total mass, 92 per cent of the debris found in the patch consists of objects larger than 0.5cm, and three-quarters of the total mass is made of macro and mega plastic," the organisation Ocean Clean Up reported.

"However, in terms of object count, 94 per cent of the total is represented by microplastics."

Recent modelling by consultancy firm Dalberg for the World Wildlife Fund estimated the lifetime cost to society, economies and the environment of plastic produced.

A Frozen balloon and a plastic bucket was seen in the near-inaccessible area. Photo / YouTube

That figure is $5.34 trillion (US$3.7t) – for a single year.

"This is the first time we have seen such a clear assessment of some of the unaccounted costs being imposed by plastic pollution on society and they are a burden that is too high to bear – both for people and the environment," WWF International director-general Marco Lambertini said.

Without urgent action, that cost is on track to double by the year 2040 as plastic production increases.