The amount of ocean plastic washing up on beaches around the world could be underestimated by up to 80%.
The shocking figure comes from new research being carried out in Australia.
Dr Jennifer Lavers, from the Institute of Marine and Antarctic Studies at the University of Tasmania, said current pollution data could represent just "the tip of the iceberg" of what is really in the environment.
Her study compared the number of plastic items found during a typical beach clean-up - the main source of data for estimates about the quantity of waste on coastlines - with the amount subsequently identified by more thorough surveys of the same areas.
Initial results of numerous comparative experiments suggest a typical beach clean-up - even if carried out by multiple individuals covering the same area - on average only identifies between 20% and 25% of the waste that is actually present on the surface.
"The reality of the plastic situation is that we are only skimming the surface," Dr Lavers said.
"What is really truly out there: we don't have the complete picture; there are big gaps in our understanding."
Scratching the surface
Dr Lavers and her research team have been carrying out detailed plastic surveys on the remote Australian Territory of the Cocos Keeling Islands in the Indian Ocean - an isolated archipelago which is home to around 600 people.
Ocean currents bring tonnes of plastic waste to the islands' shores from as far away as Sri Lanka, the Maldives, Indonesia, Malaysia, Thailand and China, presenting a major challenge to the local community.
A child plays on the seafront Muara Angke, Indonesia, 11-02-2016. Image: Rahmad Azhar Hutomo/Pacific Press/ABACAPRESS.COM
Aaron Bowman, Shire of Cocos CEO, is responsible for managing the municipal services and waste management on the islands.
"We can clean the beach one day and it is back again, and it is not from the waste we've produced," he said.
"It is a huge problem. We're a tiny little island in the middle of nowhere. We just can't tackle that ourselves."
Remote islands
Aside from the amount of waste washing up, it is the remote nature of the Cocos Keeling Islands which makes them of particular interest to scientists and anti-pollution organisations.
While accurately measuring the amount of plastic in the vast expanse of the deep oceans is almost impossible, measuring the quantities and kinds of trash washing to shore on these remote islands is an indicator of the likely density of waste at sea, due to the fact so little of it is generated by the local residents themselves.
If - as Dr Lavers believes - the amount of plastic present on beaches has been underestimated, the likelihood is the amount of plastic in the ocean itself has been too.
Her research on the Cocos Keeling Islands was carried out alongside a major citizen science project organised by a coalition of Australian environmental groups and government agencies.
Volunteers flew in from around the country to carry out beach clean-ups on the islands and collect information for the Australian Marine Debris Initiative (AMDI) database.
The database was launched in 2004 and now contains more than 7.5 million entries.
Activists collect garbage at the Baltic Sea beach of Rostock, Germany, 20-11-2015. Image: Bernd Wüstneck/DPA/PA Images
The information from across Australia indicates that around three-quarters of all the waste on the nation's coastlines is plastic.
In just a few days on the Cocos Keeling Islands, the volunteers covered around two miles of the archipelago's beaches and collected over 50,000 items, which represented just over two tonnes of ocean waste - 80% of which was plastic.
Tackling the source
Each item found was categorised, searched for manufacturer details or barcodes, and logged in the database.
"If all we do is clean-up, that's all we'll ever do," said Heidi Taylor, director of the marine debris organisation Tangaroa Blue, which runs the AMDI database.
"What we try and do is identify how the item actually ended up in the environment," she added.
"In some cases it's from the person that designed it, used it or made it. In other cases it's a consumer littering issue. So once you understand that release, you have the opportunity to prevent that," she added.
Worse than feared
Across the world, information logged from volunteer beach clean-ups provides the backbone of data regarding how much plastic is washed up on shore.
But the provisional findings of Dr Lavers' research draw into question the accuracy of the headline numbers currently used to inform political policy decisions.
For example, in 2014 the Commonwealth Scientific and Industrial Research Organisation estimated there were around 124 billion pieces of visible plastic on Australia's beaches.
Dr Lavers believes the true figure, for Australia alone, is more likely to be over 600 billion pieces.
Campaigners hope this research may inject a greater urgency into debates over how to respond to the global ocean plastic problem.
Ms Taylor added: "We need government to start accepting how big a problem this is going to be in the future.”
"This is the next climate change, and nobody's thinking that it's going to be as bad as it is.
"If we start looking at communities like the islands here, that rely so heavily on seafood, and that [seafood] is contaminated by plastics and chemicals that are in the ocean, this is going to be not an issue about saving turtles, this is going to be a human health issue, and that will be a game changer."
