Forty subterranean networks tunnel beneath Rome's southeastern periphery, holding the remains of half a million Christians, Jews, and pagans interred between the second and fifth centuries. The Catacombs of San Callisto alone extend across fifteen hectares and descend four levels, containing sixteen popes and fifty martyrs within galleries that stretch twenty kilometres. Volcanic tufa permitted excavation without collapse; Roman law forbade burial within city walls. The faithful carved loculi — body-length wall niches sealed with marble slabs and inscribed with names, anchors, fish, doves. Rome catacombs tour access centers on three principal sites along the Via Appia Antica. San Callisto, the largest and most organised, opens six days weekly under Pontifical Commission oversight. San Sebastiano preserves the only catacombs that remained accessible through the medieval period; its upper basilica marks the temporary resting place of Peter and Paul during third-century persecutions. Domitilla extends furthest, incorporating a second-century family hypogeum later expanded into a communal necropolis, with frescoes of the Good Shepherd and Veneranda's veiled portrait still legible after eighteen centuries. Guided visits descend ten to fifteen metres through hewn stairways into lamp-lit corridors. Temperature holds constant at twelve degrees. Cubicula — small burial chambers branching from main galleries — display ochre frescoes of Jonah, Noah, Lazarus risen. Inscriptions name trades: bakers, cobblers, soldiers. The Crypt of the Popes in San Callisto contains Greek epitaphs carved in Damasan script by order of Pope Damasus I in 366. Loculi stack five or six high; infant graves occupy lower rows. Light wells pierce upper galleries where collapse opened surface access. Rome's catacombs entered scholarly attention in the sixteenth century when Antonio Bosio mapped thirty-six miles of tunnels. Excavation and restoration work continued under Giovanni Battista de Rossi in the 1850s, establishing the modern chronology of Christian funerary art. The Pontifical Commission for Sacred Archaeology now manages all five open sites. Visitor access remains restricted to guided tours; photography prohibitions preserve fresco pigment. The tradition of underground burial ended with Alaric's sack in 410 and subsequent Visigoth depredations, after which bodies returned to surface churchyards and the catacombs became pilgrimage destinations for relic veneration rather than active necropolises.