A catacomb was an underground cemetery made up of galleries.
On Alfava Metraxis, the team led by Octavian had to cross a network of catacombs (which he called "Maze of the Dead") to reach the Weeping Angel they were intended to neutralise. (TV: The Time of Angels)
On Adamantine, Graham O'Brien compared the tunnels underneath Diamond City to the catacombs in Rome, which according to him were built so that the Christians could hide from the authorities. (PROSE: Molten Heart)