7. Strategic and security applications

In many wars, the underground has played an important role: sappers, i.e. military miners (part of France's Arme du Génie), have dug under fortifications to destroy them (undermine the walls), and this experience has led them to dig hidden galleries and chambers. The primary motivation for strategic installations, both civilian and military, is to prevent not only entry, but also approach and, if possible, sight. The second is to provide shelter for the cannons designed to control passage.

Many promontories have been extensively excavated for this purpose, from the Bastille in Grenoble to the Rock of Gibraltar, two almost urban examples, unlike most of the others cited in this paragraph. More superficial, trenches are shelters against shrapnel (whose trajectory is rather horizontal), often combined with small underground chambers, on the World War I front. Underground...