The latest issue of The New Yorker has an interesting piece titled "The Invisible City Beneath Paris". While I was aware of the Catacombs and had once seen a video showing that the subterranean area was larger than I thought, until I read this article I was not aware of the large extent of this area beneath the city, which is described as "two hundred miles of galleries, rooms, and chambers."
I think I'm not supposed to provide a link to this, but if you can access the current issue, it may be of interest to those who find Paris fascinating.