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leakage of petroleum onto the surface of a large body of water. Oceanic oil spills became a major environmental problem in the 1960s, chiefly as a result of intensified petroleum exploration on the continental shelf and the use of supertankers capable of transporting more than 450,000 metric tons (500,000 tons) of oil. Thousands of minor and several major oil spills related to well discharges and tanker operations are reported each year, with the total quantity of oil released annually into the world's oceans exceeding 1,000,000 tons (907,000 metric tons). The costs of such accidental oil spills are considerable in both economic and ecological terms. Oil on ocean surfaces is harmful to many forms of aquatic life because it prevents sufficient amounts of sunlight from penetrating and also reduces the level of dissolved oxygen. Moreover, crude oil renders feathers and gills ineffective, so that birds and fish may die from direct contact with the oil itself. Accidents to supertankers and to underwater wells and pipelines may be the cause of major oil spills, but the unintentional or negligent release of used gasoline solvents and crankcase lubricants by industries and individuals greatly aggravates the overall environmental problem. Combined with natural seepage from the ocean floor, these sources add oil to the world's waterways at the rate of from 3,900,000 to 6,600,000 tons (3,500,000 to 6,000,000 metric tons) a year.