seattlelimoforhire is spot on with his info, I've just added the
Catacombs location on a map for your convenience and I agree with everyone who says that the catacombs are well worth a visit.
You need to keep two things in mind though:
1) the tombs were built out of necessity and not as ornaments: hence the bones are arranged more in a bricklaying than a respectful fashion (think femur walls).
2) The place has humongous queues and you need to arrive early to make it in (I once queued 1:30h to get inside). It's mostly due to inefficient ticketing and once you're inside it should be empty and eerie enough.