Imagine, as you take a dip this summer, that you could pour all the world’s oceans through a very fine sieve. What would you find?

Well, first, you would probably notice that the old saying is correct: there are still plenty of fish in the sea. Numbers may be dwindling, but more than 22,000 fish species swim around in the earth’s waters, and they’re joined by nearly double that amount of crustaceans – lobsters, barnacles, krill, that sort of thing. You could tot up about two million whales, as well as 18 types of seal and 17 types of penguin. You would find that the sea-turtle family has seven distinct members, while manatees have just three. You would count a lot of things, in short, but there is one outstanding component that doesn’t belong there. Littered throughout the ocean, found in all shapes and all sizes, would be 5.25 trillion pieces of discarded plastic. And that is a problem.

It’s all our fault, of course. According to conservative estimates, humans dump eight million tons of waste plastic into the earth’s water system every year – the equivalent of five full shopping bags per foot of the world’s coastline. Once it’s in there, some will end up back on our shores, turning pristine beaches into hazardous tips after just one high tide. Some of it sinks straight to the seabed, but the majority stays afloat, hostage to the currents, and gets slowly dragged into one of five major ocean gyres (vast, swirling vortexes of water and wind found in the north and south of the Pacific and Atlantic, and one in the Indian Ocean) around the world. Large items – a jerry can, say, or a fishing net – can remain intact for decades, bobbing around, occasionally trapping animals, before eventually suffering the same fate as all plastic: succumbing to the conditions and crumbling into tiny pieces. These microplastics are chemical pollutants, but they’re also bite-sized. Fish, seabirds and other marine life mistake them for food, swallow them, and repeat that until it kills them.