I have mentioned in the past that a good part of my novel, The Battle of Bellicourt Tunnel, (not the complete title) will take place underground.
The extent to which caves, underground quarries, tunnels, and dugouts deep in the earth were used by the several armies that fought in World War I. They were fine places to hide soldiers from the enemy’s view, store ammunition, guns, and supplies. Several battles depended on them for the success (on the one side) and failure (on the other.) Two more images from an underground hospital add to an appreciation for the way the terrain and the earth itself served in the war.
Many of the buildings throughout the region, churches, palaces, and businesses, used the stone quarried underground from the Middle Ages on. Their presence was well known to local inhabitants.