Coal formation and mining. Artwork showing stages in the formation of coal and its eventual mining. Coal is a fossil fuel formed from dead plant material. In the Carboniferous era, between 345 and 280 million years ago, plants in swampy areas (left) died and sunk to the bottom of their swamp. Underwater, in the absence of oxygen, they did not decay but got compressed by the weight of more dead plants and sediment on top of them. Over millions of years, this formed peat (brown subsurface layer, second panel). Further sedimentation from seas (third panel) compressed the peat into coal (black layer). When near the surface, the coal can be extracted by open cast mining (right).