Because of their belief in a bodily resurrection, which was to occur at Christ's return, expected very soon, proper burial loomed large in the minds of early Christians.  Roman Christians joined with other citizens in burying their dead along subterranean passages underneath the city.  When Christianity finally triumphed in Rome about 400 AD, the custom of catacomb burial ceased.  Knowledge of the catacombs was soon lost, until they were rediscovered in 1578.