La Cité Souterraine de Naours Lille, Flanders & The Somme

Believed to date from the 3rd century AD, this extraordinary underground 'city' of quarried tunnels was started by the Romans and expanded over the centuries, incorporating 28 galleries and 300 rooms, including three chapels, multiple town squares, a bakery (with working ovens) and livestock barns. Only rediscovered in 1887, it was used by Allied forces in WWI, and as a Nazi HQ in WWII. The temperature below ground is a constant 9.5°C; bring a jacket (and a torch/flashlight). Soldiers' graffiti from both wars can be seen throughout the cave system. Guided tours are in English and French. The site is 18km north of Amiens.