Prof. Franz Kiesling, Das Ratsel der Erdstalle, Armanen Verlag, 1925

This is a study of the strange, artificial cave systems found in Hungary, Germany, France and Spain. The caves are usually 1.30 meters high and in them are hewn benches 30 centimeters high. Tunnels that are often only 35 centimeters high connect many of them.
For whom were these caves meant? Who built them?