Sources of coal can be found in mines. Coal is a combustible black or brownish-black sedimentary rock, formed over millions of years from the remains of plants and animals. It is primarily used as a fuel source for electricity generation and steel production. Mining for coal involves the extraction of coal from underground or surface mines. The most common type of mining for coal is surface mining, which involves removing the topsoil and extracting the coal from the exposed seam. Underground mining is also used to extract coal, but it is more expensive and dangerous than surface mining.