Large and small craft significantly pollute both inland and coastal waters by dumping their untreated sewage. Oil spilled accidentally or flushed from tankers and offshore rigs (900,000 metric tons annually) sullies beaches and smothers bird, fish, and plant life. In 1978 in one of the world's worst single instances of water pollution, the Amoco Cadiz broke in two on the coast of Brittany, France, and spilled 1.6 million barrels of oil, causing great environmental destruction. Oil well blowouts during offshore drilling, such as the 1979 Ixtoc 1 blowout in Gulf of Mexico off Mexico and the 2010 Deepwater Horizon blowout in the Gulf of Mexico off Louisiana, have also caused severe oil pollution. In addition to its direct damage to wildlife, oil takes up fat-soluble poisons like DDT, allowing them to be concentrated in organisms that ingest the oil-contaminated water; thus such poisons enter the food chains leading to sea mammals and people (see ecology).
Both DDT, which has been banned in the United States since 1972, and PCBs are manufactured in many parts of the world and are now widespread in the Atlantic and Pacific oceans. In addition, tarry oil residues are encountered throughout the Atlantic, as are styrofoam and other plastic rubbish. Plastic bits litter sections of the Pacific and Atlantic, accumulating in greater concentrations to form "garbage patches" where the currents are slack. Garbage, solid industrial wastes, and sludge formed in sewage treatment, all commonly dumped into oceans, are other marine pollutants found worldwide, especially along coastal areas.