The vast majority of plastic in the world's oceans is hidden below the surface, with millions of tons being buried in the seabed every year, new research suggests.

Scientists have long been puzzled as to why real-world surveys cannot account for all the plastic known to be flowing into the oceans every year.

Now the first computer models to track plastic have revealed more than 99.99% of it is out of sight.

Image: Just 246,000 out of the 393 million tons of plastic in our oceans is on the surface

Researchers from Newcastle University say most types of plastic sink in seawater, with even plastic that floats eventually becoming heavy from the growth of algae.

Of more than 393 million tons of plastic in the oceans, just 246,000 tons is on the surface. The rest is suspended in the water, lying on the bottom, or buried in the seabed.


Calculations show that between five and 36 million tons a year is being deposited in the sediment every year.

Image: More plastic is set to be produced in the next five years than in the whole of the 20th century

Alethea Mountford, the PhD scientist behind the research, told Sky News: "Even if it gets down into the deep, that isn't to say that it's gone and we can forget about it because it's not where we can see it anymore.

"There are loads of organisms still living on the sea floor, in the sediments, so they can eat the plastics.

"And we know there are chemicals released from plastic that can harm organisms like that - their reproduction, their feeding, it can make them starve to death."

Image: Alethea Mountford of Newcastle University

The computer model was based on known sources of plastic pollution around the world, the buoyancy of the different types of plastic, and the movement of ocean currents.

It reveals there is likely to be an as-yet-undocumented "garbage patch" of floating plastic in the Gulf of Guinea, off the coast of Nigeria.

And it also suggests there are hotspots of plastic in the deep ocean trenches.

The study was presented at the conference of the Challenger Society on Marine Science at Newcastle University.

Image: Cleaning up our beaches is a key step in helping the oceans

The Ocean Cleanup organisation is currently towing an experimental boom out to the Great Pacific garbage patch off the coast of California. It believes a fleet of the devices could clean up 50% of the plastic in five years.

But the new research suggests that would still represent a tiny fraction of the problem.

One of the world's foremost experts on ocean plastic, Dr Erik van Sebille, told Sky News that there were better and cheaper ways of removing plastic.

He said: "Beach cleans are a very effective way of cleaning up the ocean because beaches are not an end of plastic.

"If you see plastic along the strand-line, that means the next high tide or the next storm might pick up that plastic and put it back into the ocean.

"So by taking the plastic when it's on the beach, you prevent it from going back into the ocean and remove it from what the marine system."

Cleaning up the deep sea would be prohibitively expensive, even if it was technically possible. The new research underlines the need to use less plastic, recycle more, and prevent it from reaching the ocean.

But based on current trends, more plastic will be produced in the next five years than in the whole of the 20th century.

:: Sky Ocean Rescue aims to help everyone make simple everyday changes to stop the world's oceans drowning in plastic.

:: To find out more about the issues affecting our oceans and how to #PassOnPlastic visit skyoceanrescue.com.