I had never been to the catacombs before, it is an odd place. In 1785, it was decided to close the Cimetière des Innocents for sanitary reasons, a ten centuries old cemetery! In 1786, they started transfering all the bones to an old quarry, just underneath Paris, and named it Catacombes refering to Rome's antique cemetery. It took two years to do so and they kept on bringing bones from other cemeteries as well. They use to do it at night with a procession of priests. They stopped bringing bones in 1814.
From 1787, this place intrigued people and Kings went down there. Now a two kilometres (1.4 miles) long visit is in place. You go down 180 steps and walk along corridors until you arrive in 'l'empire de la mort' where all the bones are lined up. Avoid it if you are too tall (Tom's head was against the ceiling in most corridors) or if you are claustrophobic (you have to go all the way, 45 minutes walk, to go out). The sheer number of bones and skulls down there is overwhelming, thousands of them. I overheard people talking, saying they were shocked, disgusted, well, I mean what did you expect, don't go down there! I wonder if they feel the same going to a museum to see an Egyptian mummy. I actually feel more disturbed by a mummy. I feel more removed from bones, less real. I am actually quite amazed to think some of those people live in 953, centuries and centuries ago, imagine the stories! But I must say that after rooms and rooms of bones I was starting to feel uncomfortable, more and more thinking of the people who lined them up, surely that must mess you up, building walls of bones, lining skulls, nights after nights.
On a more technical matter, avoid weekends if you can. We went there midweek and it was almost empty, you go at your pace and it's a special atmosphere. When we walked passed it on a Saturday, hundreds of people were queueing (they can only let 200 people at the same time and it is a 45 minutes walk). Also beware of the guards sitting in dark corners, round corridors, one almost gave me a heart attack!