By David Shukman
07 September 2018 Science & Environment
When a Dutch teenager went swimming in the sea in Greece seven years ago, he was shocked to see more plastic than fish.
In fact, Boyan Slat was so appalled by the pollution that he soon started to campaign for the oceans to be cleaned up.
For a long time, few people took him seriously. Here was a university drop-out with a far-fetched idea that surely could never work.
But this weekend, backed by major investment and some massive engineering, a vast plastic collection system will be towed out of San Francisco Bay.
Until now, the focus of plastic litter campaigns has been on beaches, with volunteers all over the world lifting bags and bottles from shorelines.
Never before has anyone gone further by trying to clear the stuff from the middle of an ocean and, despite sea trials and computer modelling, no-one knows if the experiment will work.
Some experts worry that the effort is a distraction from the more pressing task of stopping more plastic getting into the sea in the first place, and that the operation may cause real harm to marine life.