Crude oil being poured into a beaker. This is a naturally-occurring mixture of hydrocarbons and small amounts of compounds of sulphur, nitrogen and oxygen. Its exact composition is variable, and it is mined from subterranean sources. It can be refined to yield the fossil fuels used to run cars and aeroplanes, as well as yielding many products used in the chemical industry. The oil is a result of the deposition of organic material from living organisms many millions of years ago. It supplies much of our energy needs, but the reserves of oil and other fossil fuels are finite and they also contribute greatly to atmospheric pollution.