You may have a coal fire in you house, or you may know someone who has, and you probably don’t think twice about putting lumps of black coal on to the fire to keep you warm. But did you know that coal is made of fossilized plants? About three hundred million years million years ago, the land was very swampy and covered with thick forests. As the trees died, they fell into the water of the swamps, which was very acid and stopped the tree from rotting. The dead trees formed a thick layer on the bottom of the swamps, and the swamps themselves were later flooded by the sea. A thick layer of sand built up on the top of the wood, and pushed it down so hard with its weight that the wood gradually hardened and became what we call coal.
These days, it is still a very hard and dirty job to mine coal, and a dangerous job too. Tunnels are dug deep underground to where the coal is buried in layers called seams. The coal is packed so tightly that powerful machines have to be used in order to get it out, and there are sometimes accidents in coal mines which happen when the tunnel roofs cave in. Luckily these don’t happen very often, but you might give the miners a thought the next time you throw a lump of that coal on to your fire.