Coal, crude oil, and natural gas are all considered fossil fuels because they were formed from the fossilized, buried remains of plants and animals that lived millions of years ago. Because of their origins, fossil fuels have a high carbon content. Many fossil fuels must be refined before being used. Years ago, when prehistoric animals and plants died, layers of rock and dirt gradually buried them. ... These fuels are called fossil fuels, since they are formed from the remains of dead animals and plants.