The Paris Catacombs is open to the public and can be visited. The only thing that you have to do is find the right ticket and tour wherein you will be most free to visit. The last time of the day when you can enter is at 7 PM.
Some like to go here at this time to increase the scare factor of the place. The catacombs are known to be very deep and they are very long. There are only a few spots of the catacombs that are available to the public. The bones that can be seen in the catacombs are the bones of the Parisians from years before.
Yes, provided you are adult or with one, mobile and not of a nervous disposition and are not carrying any kind of luggage. Opening hours are Tuesday to Sunday, and the last entry is at 7.30 p.m. Scary - are you sure you want to go! Don't steal the bones! It'll cost you $13 or $17 for two. Go with a friend!
You'll be treading the same paths as Napoleon, another tourist, although the catacombs are hundreds of years older. Originally it was a cemetery under the Roman Appian Way.