three-quarters of a mile deep. Of course, it does not pay to dig a
very deep mine except to reach thick coal beds.
A shaft or pit mine is somewhat like a giant anthill. A central shaft reaches down to a coal bed. The miners must cut long passages, like the runways in an anthill, from the bottom of the shaft hundreds of feet through the coal. In the anthill, however, the galleries, or tunnels, run in many directions, up and down as well as sideways. In the coal mine the galleries follow the coal bed and can run no farther up or down than the coal.
In an anthill a central shaft as large around as a lead pencil may lead into a nest spreading over several square feet. Just so, a coal shaft measuring fifteen by thirty feet or less at the top may be the gateway to an underground city spreading over several acres. One mine in this country has an underground spread of about ten square miles.
How do the miners find their way about in their man-made anthill into which no sunbeam ever enters? Why does the huge weight of the earth above the underground city not settle on it and crush it? Let us go and see. We shall have to charter an imaginary airplane, for we are going to travel far in a short time!