Fossil fuel is an energy source formed by natural processes from plants which undergo an anaerobic partial decomposition, which contains energy originating in photosynthesis. Another theory came up that has a clearer explanation of this is that fossil fuel was created from the fossilized remains of the dead plants by exposure to heat and pressure in the earth’s crust millions of years. Furthermore, fossil fuel is compressed to become natural gas, petroleum, kerosene, propane and coal.