Forty kilometres of tunnels spiral beneath the Via Appia Antica, holding half a million burials carved into tufa between the second and fifth centuries. The Catacombs of San Callisto alone shelter sixteen popes and dozens of martyrs, their loculi stacked four deep along corridors that drop three storeys below the Roman countryside. Early Christians excavated these networks not as hiding places — persecution was sporadic, not continuous — but as communal necropolises where the faithful could gather for anniversary rites without the expense of surface land. The soft volcanic stone allowed rapid expansion, and by the fourth century Rome's suburban roads were honeycombed with subterranean cemeteries, each managed by a different Christian community or funerary guild. Today the catacombs remain among the most intact witnesses to early Christian life, their frescoes and inscriptions preserving a theological vocabulary still forming when Constantine legalised the faith. The oldest images — a fish, an anchor, a shepherd — predate the cross as the dominant symbol, and the Greek epitaphs outnumber Latin ones in the earliest galleries, evidence of a congregation still weighted toward the eastern Mediterranean. Wall paintings in the Cubiculum of the Sacraments depict baptism, Eucharist, and resurrection in a visual grammar that would calcify into iconographic convention only centuries later. The catacombs were abandoned by the sixth century, their relics translated to basilicas within the city walls, and the tunnels lay forgotten until Antonio Bosio began systematic exploration in the 1590s. Nineteenth-century archaeologist Giovanni Battista de Rossi mapped San Callisto's papal crypt, recovering inscriptions that anchored the legendary bishops to historical ground. Rome underground tours navigate a fraction of the total network — most galleries remain closed to preserve the frescoes and prevent structural collapse — but the accessible sections convey the scale and intimacy of early Christian devotion. The Catacombs of San Callisto open daily except Wednesday, with guided visits required to enter the underground chambers. Standard admission is ten euros, and the site lies three kilometres south of the Aurelian Walls along the ancient consular road. Morning slots between nine and noon offer cooler temperatures and thinner crowds, and the last entry closes at five in the afternoon, leaving an hour before the site shuts at six. The tour descends through three levels, passing papal tombs and the crypt of Saint Cecilia before surfacing near the basilica that marks the original entrance. No photography is permitted below ground, a rule enforced to slow the bleaching effect of camera flashes on pigment that has survived sixteen centuries in darkness.