"I'm not through my course yet," reminded his brother, "but I think I can answer that.  The air surrounding the earth is a great belt forty or more miles through and is of an even thickness.  As our globe sweeps through it, the lower stratum of air naturally sinks down into the valleys and like depressions.  This action pulls down the upper stretches of air, thus creating what are termed 'air-pockets' or 'air-holes.'  Very dangerous they are, too."