This is a picture of the inside of the San Callixtus Catacombs (We were not allowed to take photos during our tour inside).
The catacombs are made up of underground tunnels in the form of a labyrinth.  The corpses, in imitation of Christ, were wrapped in a sheet or shroud and placed in the loculi.
After the issuing of the edict of Milan by the Emperor Constantine in 313 AD, the catacombs began to be replaced by regular cemeteries as the Christians were now free to buy plots of land, as well as free to build churches and profess their faith freely.