hink about pollution of the sea, and big spectacular oil spills spring to mind. But in fact they are a minuscule part of the problem, responsible for only 5 per cent of the oil that gets into the sea, itself just a small contribution to overall marine pollution.

Normal ship operations - such as discharges of oily ballast water from fuel tanks - pose much more of a problem. But, in fact, by far the biggest culprits are cars and other land vehicles: their used engine oil - and oily water running off roads - provides much the largest percentage of oil pollution in the ocean, after reaching it through drain outlets and sewers.

In all, 80 per cent of sea pollution comes from the land. The greatest load comes from untreated, or lightly treated, effluent from sewers, burdened both by human waste - with harmful bacteria and viruses - and pollution from industry and intensive agriculture.

Human sewage, agricultural fertilizers and other nutrients stimulate great blooms of algae. Sometimes these contaminate shellfish with dangerous poisons, and they always suck oxygen out of the water, often suffocating fish and other marine life. This contributes to the growth of 'dead zones' in the seas and oceans. UNEP has identified 146 of them (see map): their numbers have doubled every decade since the 1960s. Two of the biggest - at around 70,000 square kilometres - are in the Gulf of Mexico and the Baltic Sea. Meanwhile, chemicals like polychlorinated biphenyls (PCBs) accumulate in fish.

Some 95 per cent of the world's fish catch comes from the coastal waters where we dump our waste. In all, polluted coastal waters cost the world economy $12.8 billion a year in death and disease.