The Citadelle Souterraine is the underground supply city that kept the French army at Verdun alive through 1916. The seven kilometres of tunnels — bakeries that produced 28,000 loaves a day, hospitals, ammunition stores, electric lighting, even a chapel — were the nerve centre of the entire battle.
The visit is unusual: you ride through the tunnels in small driverless rail-cars while audiovisual scenes are projected onto the walls and at intervals into reconstructed chambers. It's pitched at families but the historical content is solid, and the scale of what was built underground is genuinely surprising. Allow 90 minutes.