The soil beneath Rome (referred to as tuff) is suitable for tunneling because it is initially soft (when first exposed to air) and thereafter hardens (which prevents cave-ins). As Rome expanded during the second century of the common era (and Christians continued to be persecuted at that time), people needed more space (or secret places) to bury their dead. Catacombs became a solution.
At least forty different catacombs under the city of Rome (or nearby) have been identified. Beyond their role as burial sites, the catacombs today are important for what they tell us about Christian, Jewish and pagan art during the second through fourth centuries of the Roman Empire. (After Christianity became the state religion of Rome, in 380 A.D., the use of catacombs declined.)
This image is from the interior of a Roman catacomb. Burial places (referred to as loculi) were dug out of the walls of underground passages (called ambulacra).