Just found this on t'interweb:
http://www.aol.co.uk/video/first-world-war-cave-inscriptions-discovered-in-france-518949160/?ncid=webmail5 For the purposes of wider reporting to the uninstructed and popular world, I'm quite happy to go along with the C- word, however it is a fairly well accepted fact that many subterranean voids in that region are in fact chalk mines, many of great antiquity.
The town of Arras has a particularly fine network under its streets, dating back many hundreds of years, and I also recall visiting Les Grottes Naours near Amiens in 1980. Chalk mines both.
Enjoy
MARK