Naours just north of Amiens has what they like to call a subterranean city. It’s basically a network of 35km of tunnels dug out of the chalk and flint, to allow locals to live and hide there from the Germans during the war.
One part had a cave-in and took locals 4 years to clear they say.
You only get to see a very small part of it, I’d guess it would be repetitive anyway to see much more as it was built to hold 3,000 people. The clever priest who thought of the idea also tried to make most of the alleys mirror the layout of the town above them and even named the alleys with the same names as the streets above. This helped people find their way around and also made it seem more homely. There were tunnels in the roof that vented smoke to the surface, which came out through the chimney of one of the houses. Very ingenious!
It very much reminds me of the man-made caves in Kent built for the same reason, I can’t for the life of me remember the name of them though.
If you look at the photos you can see very black bits in the chalk. This is what the French call Silex but we call flint. They left it there as it’s very hard and helped strengthen the caverns.
This is an amazing construction designed to hold an entire village including their livestock, in complete secrecy from those above ground. The motivation and social cohesion needed to build something like this probably just doesn’t exist anymore.