here's our science editor david shukman. the shocking sight of plastic waste polluting the oceans. it is one of the most visible ways that human activity is damaging the natural world. so where has it all come from? well, most of it is carried by rivers from cities to the sea. i saw this for myself in indonesia a few years ago. the waterways were choked with plastic. it used to be thought that only the ten biggest rivers in the world where to blame, but now a study finds that many more are involved. at least 1000, according to an organisation that is trying to tackle the problem — the ocean clean—up. on one hand, it shows that the problem is actually much more vast than we used to think — it is not ten rivers, it is 1000. but on the other hand, it is stilljust1% of all the rivers in the world, so i do think it is a manageable problem.