Never heard of this before until it was mentioned in this article-
http://www.cracked.com/article_19421_7- ... ure_module
Built during World War 1, near Arras, France:
But the sneaky reason Arras caught their attention was its history. It was ancient and built over a sprawling network of cellars, tunnels and sewers, and the surrounding countryside was veined with vast underground caves where chalk had been mined during the Middle Ages.
Looks like an interesting mix of caves, mines, tunnels, and history.