How is coal formed?

Coal is essentially made from dead plant material. Around 200 lion years ago, in the carboniferous period, huge ferns grew in the tropical climate and dead plants sank in the swamp. The dead plant layers forced oxygen out and left rich carbon deposits. The pressure of this layers pressed the water out of the remains of the plants slowly. This first gave rise to brown coal. Hard coal was formed when the pressure increased further. It is found in deeper layers and is a high quality fuel. It is dug out in mines. Brown coal, on the other hand, is mined by using excavators.