In 2006 I worked in Northern France for one of the European camping companies. Awesome time! This is where I had my first taste of Urbex too!
The site was near two villages called Berny Riviere and Vic-Sur-Aisne, and the two nearest towns were Soissons and Compiegne. This area was badly damaged by both WW1 and WW2 action and there were literally dozens of underground caves which were dug during WW1 I believe. There was a front line about a mile away and we were lead to believe that they were used by the French Resistance, civvies and soldiers.
They ranged from very large (1.5km square) to smaller and many still have their original features such as, air raid sirens, WW1 cars, chapels etc.
Once I work out how to get pics on these threads then I'll be on it!