Plan of Domitilla 9. Mainly confined to two levels because of the poor quality of the tufa and its situation over an underground water channel, this catacomb, which extended for about 15 kilometers, was built around the basic nuclei of hypogea. So vast was this labyrinth of tombs that Bosio, one of its earliest investigators in 1593, lost his way. Plan of G. Palumbi (Pontificia Commissione di Archeologia Sacra, 1907).