If we step forward and enter one of the doorways to be found on either side, it will introduce us to a small chamber, twelve or fourteen feet square perhaps. If there is nothing but graves cut in the walls of this chamber, just as in the galleries, we may safely conjecture that it was only a family vault. But if we find a bench hewn out of the rock all round the room, together with a chair (or perhaps two) similarly excavated, we can hardly be wrong in supposing that the room was once used as a place of assembly, whether for purposes of public psalmody or of religious instruction. Or, if the principal tomb in the chamber be shaped like an altar, if two or three chambers open out of one another, and if one of these have an absidal termination, with a chair at the end and a low seat running round the sides, such as may still be seen in some of the old basilicas above ground for the accommodation of the bishop and clergy, no one can justly accuse us of rashness if we suspect that we stand in a place that was provided for the celebration of the Christian mysteries in days of persecution. [Illustration: _Arcosolium, or altar-shaped tomb._] [Illustration: _Part of Catacomb of St. Agnes._] If we were sufficiently bold to leave the gallery which we first entered, and to pursue our way further into the interior, we should soon lose ourselves in some such labyrinth as is here represented. This diagram is a true map of a small portion (perhaps not more than an eighth) of a catacomb on the Via Nomentana, commonly known by the name of St. Agnes. It is about 230 yards long by 180 in width; yet if all the galleries in this small section of a catacomb were stretched out in one continuous line, they would make very nearly two English miles in length. And then we must remember that in almost all these subterranean cemeteries the same thing is repeated on two or three different levels—in some of them even on five levels. In the Catacomb of St. Callixtus, on the Via Appia, we can descend by a succession of staircases to five different _stories_, so to speak; and perhaps the excavators might have made even a sixth and a seventh had they not by this time come to the level of water, where any galleries they might dig would soon have become mere subterranean canals. [Illustration: _Section of the Cemetery of St. Callixtus._] But we have not even yet finished our tale of wonder. One such excavation as this would have been a remarkable work, and its history would have deserved examination. But in the hills round Rome there are forty or fifty such, of various sizes indeed, and each having its own name and history, but all evidently inspired by the same idea and forming part of the same general plan. What was this plan? For what purpose were these labyrinths of long narrow galleries and small chambers so laboriously excavated?