I remember having visited the non-public version of the catacombs when I was a 25 y.o. kid. I was brought, by a pair of friends, directly down into the catacombs. We entered via a sewer-like plate that they removed with appropriate tools. We then walked in mud and water, sometimes climbed on stones, etc., we also met a bunch of crazy people organizing parties, pancake fairs, and so. Yes, all of this inside! This was damned cool and I have a lot of memories of that. I hope I could find back the pictures and share them if you are interested. Note that I am probably too old for doing that again today ;)

I have however never been to the official museum. Should do it one day.

pS: Please let me know the next time you and @teamsteem will be in Paris. We could get some drinks :)