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.

　　水的污染是由于工厂和城市的废物引起的。大洋能够自身清除污染，但是某些海域一旦被污染，却没有自身清污的能力。位于欧洲和非洲之间的地中海就是一个例子。它只在西部有一个狭窄的通往大洋的入口。四分之一的地中海海岸被污染了，而且由于水中带有大量病菌，所以在水中游泳就不再安全了。大多数地方，吃鱼也不保险。