in the first century after Christ, Christians did not have their own cemeteries,If they owned land, they buried their relatives there, otherwise they resorted to common cemeteries, where pagans too were buried.In the first half of the second century, as a result of various grants and donations, the Christians started burying their dead underground. That is how the catacombs were founded. Many of them began and developed around family tombs, whose owners, newly converted Christians, did not reserve them to the members of the family, but opened them to their brethren in the faith. With the passage of time, these burial areas grew larger by gifts or by the purchase of new properties, sometimes on the initiative of the Church itself. Typical is the case of Saint Callixtus: the Church took up directly the organization and administration of the cemetery, assuming a community character. With the Edict promulgated by the emperors Constantine and Licinius in February 313, the Christians were no longer persecuted. They were free to profess their faith, to have places of worship and to build churches both inside and outside the city, and to buy plots of land, without fear of confiscation. Nevertheless, the catacombs continued to function as regular cemeteries until the beginning of the fifth century, when the Church returned to bury exclusively above ground or in the basilicas dedicated to important martyrs. How arrive there from Home? easy: out of the Entrance, there is the Bus 660, with few stops you arrive near the Catacombs.... Fot other info and tickets :
http://www.catacombe.roma.it/en/index.php
http://www.catacombe.roma.it/en/index.php