We treat the oceans like a giant dustbin, dumping huge amounts of waste in them everyday, often by way of rivers, drains and outflow pipes. This pollution includes human sewage and domestic waste water, factory outflows of acids and poisonous metals, engine oil from roadside drains and garages, farm chemicals washed off the land by rain, building-site rubble, nuclear waste from power plants, and oil from wells, refineries, and tankers. The daily flow of materials into the sea also includes a million plastic items, such as bags, nets, and bits of packaging.

The oceans break up, disperse, or dissolve large quantities of waste. But there are limits. Most plastics never break down but just wash around coasts and islands for years. And there is no known way of getting rid of deadly substances such as the pesticide DDT (now widely banned) and PCBs ( a group of chemicals used in factories) once they enter the water.

More than 3 million tonnes of oil end up in the sea each year, the result of a variety of human activities, most of which take place on land. More than a third is waste oil from city drains - mainly from cars and from factories. This is carried to the sea by rivers. Some oil evaporates off the land into the air and reaches the sea via weather systems in the circulation of the atmosphere. More comes from tankers, either during cleaning or from accidental spills. Leaks at oil wells are another source, and some oil seeps naturally from the seabed.