Petroleum has been important since ancient times. The Greek historian told of its use in the form of pitch for building and road making. Petroleum is a fossil fuel. The liquid comes from the remains of plants and animals that died millions of years ago. These remains were buried deep below levels of rock over time and under great pressure. This geological process created complex molecules of hydrogen and carbon. Oil can also contain other elements. Crude oil, or unprocessed petroleum, is called sour when it contains a lot of sulphur, an impurity. Sour crude requires more refining than sweet crude, which is low in sulphur and, as a result, often more valuable. The modern history of oil started in the middle of the 1800s. At that time, a method was found to make kerosene fuel from petroleum. This kind of fuel became popular for heating and lighting.