Dear MP,
Below is a link to a Wikipedia article that lists a number of them. Decades ago I visited those of St. Calixtus and St. Sebastian, both on the Via Appia, and I was moved by them. I believe one of them was used to hide the skulls of the Apostles Peter and Paul during the severe persecution of 250+/-.
I think that of St. Sebastian has seen the most "traffic", and for that reason, is less pristine. However, I wouldn't be surprised to learn that the Italians have been busily restoring it and the others in the years since I was there!
Catacombs of Rome [
en.wikipedia.org]