A naturally occurring organic hydrocarbon fuel formed millions of years ago from the decomposition of fossilized remains of plants, trees, and aquatic life to form coal, oil, and natural gas found in rock strata. With a high carbon and hydrogen content, they (p. 156) are energy-rich having captured the energy of the sun in prehistoric times. Fossil fuels are extracted from the ground and used for power generation in power stations, as fuel for motorized vehicles, and for domestic heating. In power stations, the fuel is combusted to produce heat, which is used to produce steam from water that supplies steam turbines to drive a generator. Coal is the most widely used fossil fuel. Both coal and wood were burned in the earliest power stations. In 1920, crushed coal was first used thereby improving combustion efficiency. However, limited economical coal supplies and pollution concerns have led to a recent decline in its use. Oil and gas are also used as fuel in power stations. Natural gas, which is largely methane, produces less airborne pollution than either coal or gas. Natural gas provides a significant proportion of the UK’s electricity and is combusted directly in a gas turbine. As a finite and non-renewable energy source, fossil fuels produce carbon dioxide through combustion and are attributed to causing pollution and the greenhouse effect.