Fossil fuels are derived from fossils that were formed millions of years ago. They are fossilized organic remains that over millions of years have been converted to oil, gas, and coal. These fuels are made up of decomposed plant and animal matter. The dead and decomposed were buried under the earth for many years, the dead and decaying matter under a great amount of pressure and temperature transformed into fossils. Elements present in fossil fuels in addition to carbon are hydrogen, nitrogen and sulphur.