It is well documented that large quantities of plastic enters the oceans every year, polluting the seas, littering beaches and endangering wildlife. And that plastic pollution has been found even in the most remote regions of Earth, such as Antarctica or in the deepest parts of our oceans.
But relatively little is known about the levels of plastic nestled on the sea bed and in the deep-sea sediment at the bottom of our world.
A new study from Australia's national science agency CSIRO and published in the journal Frontiers in Marine Science on Monday, has shed some light on this plastic problem, estimating that there is 14 million metric tons of microplastics sitting on the ocean floor.
That's more than 35 times as much plastic than is believed to be floating on the surface, the study suggests.
Researchers say the work is the first global estimate of microplastics -- pieces of plastic that have been worn down by the elements into tiny fragments, smaller than 5 millimeters (0.19 inches) -- on the seafloor.
Using a robotic submarine, the team collected samples of deep-sea sediments up to 3,000 meters (9,800 feet) deep from six sites in the Great Australian Bight, up to 380 kilometers (236 miles) off the coast of South Australia.
Analysis of the 51 samples taken found that there was an average of 1.26 microplastic pieces per gram of sediment. Researchers say that's up to 25 times more microplastics than previous deep-sea studies.
Justine Barrett from CSIRO's Oceans and Atmosphere who led the study, said in a statement that, "Even the deep ocean is susceptible to the plastic pollution problem."
"Plastic pollution that ends up in the ocean deteriorates and breaks down, ending up as microplastics," she said. "The results show microplastics are indeed sinking to the ocean floor."
Scientists have called plastic litter "one of this generation's key environmental challenges" and the problem is an internationally recognized environmental issue. Plastic is a persistent pollution that hurts wildlife, the ocean itself and there's growing concern about the potential health risks it poses to humans.
The problem is only set to get worse as plastic production and pollution is expected to increase in the coming years.
One recent study suggested that even if we embarked on an immediate and globally-coordinated effort to reduce plastic consumption, there would still be an estimated 710 million metric tons of plastic that will pollute the environment by 2040.
Dr. Denise Hardesty, Principal Research Scientist and co-author of the study, said they were "surprised to observe high microplastic loads in such a remote location."
Research shows that plastic can accumulate on the seafloor by sinking through the water column or from currents that transport microplastics down underwater canyons to the deep seafloor.
"Our research found that the deep ocean is a sink for microplastics," Hardesty said. "By identifying where and how much microplastic there is, we get a better picture of the extent of the problem."
The core samples were taken in March and April 2017 from a range of depths of between 1,655 meters (5,400 feet) to 3,062 meters (10,000 feet). Researchers then scaled up their data taken off the coast of Australia to get a global estimate on the weight of microplastics in deep-sea sediment and reached an estimate of 14.4 million metric tons -- an estimate they say is conservative because the location was remote and away from urban population centers.
While their findings show a huge amount of microplastics on the seafloor, the researchers say it is a fraction of the total amount of plastic that is annually dumped into the world's seas.
Around 150 million metric tons of plastic are already floating in our oceans -- with an additional eight million tons entering the water each year, according to the World Economic Forum.
And the researchers say most of the plastic dumped into the ocean likely ends up on the coasts, rather than on the ocean's surface or floor.
The results show that it's more urgent than ever to find effective solutions to stop plastic pollution before it reaches the oceans, and to cut plastic use in general, according to Hardesty.
"This will help to inform waste management strategies and create behavioural change and opportunities to stop plastic and other rubbish entering our environment," she said. "Government, industry and the community need to work together to significantly reduce the amount of litter we see along our beaches and in our oceans."