Coal is found in the earth in beds, or layers. In some parts of the earth layers of coal-- some of them perhaps less than an inch thick, and others many feet thick-- are piled up as in a layer cake, with layers of different rocks as "filling" between.
In some places a crumpling of the earth's crust has so broken and tilted the coal beds that their bare edges stick up out of the ground. There it is easy to break off all the exposed coal to carry away. When the surface coal is gone, it is not hard to follow up a tilted coal seam by digging a "slope" mine. A coal seam standing out on a hillside may not be tilted. It may be followed up by digging a "drift" mine into the side of the hill.
In some places coal lies so near the surface that the easiest way to get it is to shovel off the earth above before digging the coal. This kind of mining is "strip" mining. In modern strip mines great electric or steam shovels first remove the earth. Then smaller power shovels scoop up the coal.
Much of the easily mined coal was used up long ago. Today most of our coal comes from "shaft" or "pit" mines, perhaps one or two hundred feet deep, perhaps deeper still. A mine is Belgium is nearly