I must admit that at first I thought catacombs were some modern art, made as an attraction for tourists.
Oh, how wrong I was...
I am not going to narrate about its history, ways of laying bodies and so on. There are detailed descriptions on Greeka and the very own site this attraction has.
I will just say it's worth visit.
Have in mind that only groups of seven people may enter for one tour inside (and you go with the guide; the entrance is not "free"), so it can happen that you wait a little, but if you are going during season, it will be a few-minutes waiting.