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The existence of oil wells has been known for a long time. Some of the Indians of North America used to collect and sell the oil from the wells of Pennsylvania. No one, however, seems to have realized the importance of this oil until it was found that paraffin oil could be made from it; this led to the development of the wells and to the making of enormous profits. When the internal combustion engine was invented, oil became of worldwide importance. What was the origin of the oil which now drives our motor cars and aircraft? Scientists are confident about the formation of coal, but they do not seem so sure when asked about oil. They think that the oil under the surface of the earth originated in the distant past, and was formed from living things in the sea. Countless billions of minute sea creatures and plants lived and sank off the sea bed. They were covered with huge deposits of mud; and by processes of chemistry, pressure, and temperature were changed through long ages into what we know as oil. For these creatures to become oil, it was necessary that they should be imprisoned between layers of rock for an enormous length of time. The statement that oil originated in the sea is confirmed by a glance at a map showing the chief oilfields of the world; very few of them are far distant from oilfields of the world; very few of them are far distant from the oceans of today. In some places gas and oil come up to the surface of the sea from its bed. The rocks in which oil is found are of marine origin too. They are sedimentary rock socks that were laid down by the action of water on the bed of the ocean. Almost always the remains of shells, and other proofs of sea life, are found close to the oil. A very common sedimentary rock is called shale, which is a soft rock and was obviously formed by being deposited on the sea bed. And where there is shale there is likely to be oil. Geologists, and scientists who study rocks, indicate the likely places for the oil drillers. In some cases, oil comes out of the ground without any drilling at all and has been used for hundreds of years. On the island of Trinidad, the oil is in the form of asphalt, a substance used for making roads. Sir Walter Raleigh visited the famous pitch lake of Trinidad in 1595; it is said to contain nine thousand million tons of asphalt. There are probably huge quantities of crude oil beneath the surface. The king of the oilfield is the driller. He is a very skilled man. Sometimes he sends his drill more than a mile into the earth. During the process of drilling, gas, and oil at great pressure may suddenly be met, and if this rushes out and catches fire the oil well may never be brought into operation at all. This danger is well known and steps are always taken to prevent it.