At different times in Earth's history, organic life abounded at the water's surface. But in the depths many plants and animals could not survive. When they died, the plants and animals sank to the ocean's depths and collected one on top of the other to make thick layers of organic substance. At some time these were changed little by little into oil and gas.

But oil deposits are also found in deserts. This is not surprising: once, deserts were at the bottom of the ocean. Think of the Arab countries (Saudi Arabia, Iran, Libya, Kuwait) where oil has been discovered. The oil used mainly as motor oil is one of the most important sources of energy of our time. It is also a primary substance from which many other products are made. Besides petrol for cars and aviation fuel, dishes, records, paint, cosmetics, car tyres and plastics are all made from oil!