The need of secrecy and the high strategic value of military installations make the use of the underground space of particular interest.
Although there are numerous cases which exemplify this use of the underground, it is normally after they have ceased all operation that public opinion knows of their existence.
Nowadays, we know the extensive use of the underground space made during the WWII and the cold war era, in form of bunkers, secret military bases and underground weapon manufacturing plants.
It is also anecdotic that the secrecy associated to these facilities has fed over the years legends of complex military facilities where secret material, on occasions even associated with extraterrestrial activity, is stored.