Rome's catacombs were forgotten so completely after the early Middle Ages that workers rediscovered one network entirely by accident in 1578, breaking through into tunnels nobody living had entered for centuries.
Burial galleries, not hideouts
The catacombs consist of extensive underground tunnel networks cut into soft volcanic rock, lined with loculi, rectangular burial niches stacked in rows along the walls, and decorated in places with early Christian frescoes depicting biblical scenes among the oldest surviving Christian art anywhere. Major networks, including the Catacombs of San Callisto, Priscilla and Domitilla, held burials across several centuries and functioned as large, organised cemetery systems rather than secret refuges.
Abandoned, then forgotten, then rediscovered
Following the removal of relics into churches within the city walls during the early medieval period, largely to protect them from raids, active use and even general knowledge of the catacombs' locations faded, and many entrances were lost from common memory. Workers accidentally broke into a network on the Via Salaria in 1578, an unexpected rediscovery that helped revive scholarly and religious interest in the tunnels after centuries of neglect.
As this entry's companion article covers
The popular but inaccurate belief that the catacombs served as hidden refuges for Christians during periods of Roman persecution is addressed directly in this entry's companion article rather than repeated here.