Campaigners have warned that British beaches are being inundated by a form of plastic pollution that looks exactly like rocks.
The so-called 'pyroplastics' are believed to be remnants of plastic that has been burnt or melted, researchers said. They have been spotted this week in Wales.
Hilary Rowlands, a founding member of Tywyn Beach Guardians in Gwynedd, told North Wales Live: "It's only when you pick them up, and feel how light they are, that you realise they are not stones at all.
"There's not a single beach I've combed where I haven't come across them. Sometimes they are covered in oil or impregnated with the toxins that come from burning plastic.
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"It's all dangerous, both to the environment and the marine life.
"The longer-term concern is that they will break down into microplastics and threaten marine food chains."
Pyroplastics look almost exactly like pebbles, and are created when plastics are heated during manufacturing processes.
Researchers began to analyse the 'rocks' in recent years after people spotted them on beaches in Cornwall – initially thinking they were real pebbles.
The lumps of plastic also weather like real rocks, and shed microplastic into the environment.
Some of the lumps could be as much as half a century old, according to Andrew Turner of the University of Plymouth.
Turner writes: "Pyroplastics are derived from the burning of plastic. Some may look like various burnt pieces of plastic amalgamated together, while others look remarkably like pebbles once they have been eroded down by the elements.
"They have probably been in existence since we started burning plastic to dispose of it (perhaps 80 years or so). Some of the now restricted chemicals we find in pyroplastics suggest they have been around since at least the 1960s.
"Burnt plastic on beaches is likely to be derived from many sources, including burning waste on the beach itself, collapse of old landfill sites, historical burning of waste at sea and contemporary burning of plastic waste on small island states."
Pyroplastics are found worldwide, with samples having been located on Atlantic beaches in Spain and the Pacific beaches of Vancouver.
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