water pollution is caused by waste from factories and cities. oceans are able to clean themselves, but certain seas,once they become dirty, are not able to do so. one example is the mediterranean which lies between europe and africa. it has onlyeone narrow entrance to the ocean in the west.
on quarter of the shores of the mediterranean are polluted and are no longer safe for swimming, as a lot of diseases are present in the water. in most places it is not safe to eat the fish.
lakes also have the same problems. lake baikal in asia was once the cleanest in the world, with over 700 different kinds of plant and animal life. now, however, the waters of this great lake, which is also the world＇s deepest (over 1,740 metres), have been dirtied by waste from a chemical factory.
in 1989 an oil tanker hit a rock off the northwest coast of alaska. 35,000 tons of oil poured into the sea. the accident was one of the worst in history. more than 34,000 birds and 10,000 animals were killed. 4,800 square kilometres of ocean were polluted.
推荐阅读：
WhyareChildrenLackingSleep?(为何年少总是缺觉?)
