Even though I lived pretty close to the location of the Catacombs (an underground ossuary with the remains of 6 million people) when I studied in Paris, it was one thing I never got around to doing. Mistaaaaake. It is super awesome.
One thing though...the lines are pretty crazy. They only let in a certain number of people at a time, so you could be waiting for hours. One way around this is to take a Skip the Line Tour with Localers.com like we did. There are pros and cons to this. As mentioned, the greatest pro is that you get to skip the line. Another pro is that you get LOTS of information from your tour guide. However, this can also be a con. It's really awkward to stand around in the middle of these narrow passageways as people bunch up behind you, confused as to why nobody is moving. You don't get much of a chance to take in the fact the you're FUCKING SURROUNDED BY HUMAN BONES.
So, I was a jerk and left the tour. Not a nice thing to do, but totally worth it. It was just me and the skulls as my dad and brother languished behind being polite.
On the way out out you have to go up a seemingly endless spiral staircase, so this attraction is only for the at least semi-fit. Some more pictures:
So metal. Highly recommended.
Catacombes de Paris
1 avenue du Colonel Henri Rol-Tanguy