Citadel of Verdun

Very impressive fortress right next to the city center of Verdun!
Underground galleries were dug under the citadel of Verdun between 1886 and 1893. Established and supported for 2,000 men, this underground city played an important role during the battle of 1916.
Source: tourisme-lorraine.fr/memoire-militaire/centenaire-14-18/sites-et-monuments/939000374-citadelle-souterraine-verdun

Yes, you should have seen it..it's also worth a visit
inside

The Citadel of Verdun is a symbolic and strategic site of the Battle of Verdun, nestled in stone. It will be a visit like no other during which you will walk through the underground galleries thanks to augmented reality. This unmissable memorial site immerses us in the lives of the soldiers during the war, 16 meters under the rock. An unprecedented experience!Triggered? Click through to landofmemory.eu/nl/sites-historiques/ondergrondse-citadel