These catacombs are in fact profound galleries divides on several levels.They are situated in Rome's periphery and were mainly used by the first Christians when they wanted to have some meetings.After a while they became a cemetery (I-Iv century)
You can find in San Callisto the biggest Christian's catacombs ever seen, in Rome, in fact they were created in the III century BC.
Dividedon four levels the masterpiece is the crypt where nine Popes from the first Christian age are buries.