: Coal is one of the fossil fuels. Fossil fuels are a type of fuel that are made in the process of anaerobic digestion in the absence of air. This fuel is formed by burying dead tree leaves , carcasses, and other living elements under the ground for thousands of years. [1] The process takes millions of years to produce fuel , usually 650 million years. [2] Fossil fuels contain high levels of carbon. Fossil fuels such as coal , natural gas , mineral oil, etc. [3] Fossil fuels are not found everywhere in the world. Depending on the country in which it is found.