LAYERS of coal are not always straight and level. They are often slanting, or hollow like a basin.
Sometimes they bend, so that the coal comes to the surface of the ground. Then it is easy to mine the coal by digging into a kind of tunnel.
Sometimes very deep holes must be dug to reach the coal. These holes are called shafts. Cages or huge buckets are drawn up and down these shafts by means of ropes.
If you ever visit a coal mine, you will find yourself in a new world. Everything is black.
As you go down with a rush away from the sunlight and cool air, you wonder when the journey will end.
Suppose the ropes should break! You grasp the sides of the cage tightly, and go down into the darkness.
It is not a bit as if you were gliding down into the depths of the earth, but as though the earth were rising and you were standing still.