The present descent into the catacombs that lie near the churches of St. Agnes and St. Constantia is by an entrance in a neighboring field, made, after the time of persecution, to accommodate those who might desire to visit the underground chapels and holy graves.  A vast labyrinth of streets spreads in every direction from it.  Many chambers have been cut in the rock at the side of the passages,—­some for family burial-places, some for chapels, some for places of instruction for those not yet fully entered into the knowledge of the faith.  It is one of the most populous of the subterranean cemeteries, and one of the most interesting, from the great variety in its examples of underground architectural construction, and from the number of the paintings that are found upon its walls.  But its peculiar interest is, that it affords at one point a marked example of the connection of an arenarium, or pit from which pozzolana was extracted, with the streets of the cemetery itself.  At this point, the bed of compact tufa, in which the graves are dug, degenerates into friable and loosely compacted volcanic sand,—­and it was here, very probably, that the cemetery was begun, at a time when every precaution had to be used by the Christians to prevent the discovery of their burial-places.  No other of the catacombs gives a clearer exhibition of the differences in construction resulting from the different objects of excavation.  In the Acts known as those of St. Valentine it is related, that in the time of Claudius many Christians were condemned to work in certain sand-pits.  Under cover of such opportunities, occasions might be found in which hidden graves could be formed in the neighboring harder soil.  In digging out the sand, the object was to take out the greatest quantity consistent with safety, leaving only such supports as were necessary to hold up the superincumbent earth.  There are few regular paths, but wide spaces with occasional piers,—­the passages being of sufficient width to admit of the entrance of beasts of burden, and even of carts.  The soil crumbles so easily, that no row of excavations one above another could be made in it; for the stroke of the pick-axe brings it down in loose masses.  The whole aspect of the sand-pit contrasts strikingly with that of the catacombs, with their three-feet wide galleries, their perpendicular walls, and their tier on tier of graves.

The stratum of pozzolana at the Catacombs of St. Agnes overlies a portion of the more solid stratum of tufa, and the entrance to the sand-pit from the cemetery is by steps leading up from the end of a long gallery.  Such an entrance could have been easily concealed; and the tufa cut out for the graves, after having been reduced to the condition of pozzolana, might easily at night have been brought up to the floor of the pit.  In many of the Acts of the Martyrs it is said that they were buried in Arenario, “in the sand-pit,”—­an expression which, there seems no good reason for doubting, meant in the catacombs whose entrance was at the sand-pit, they not having yet received a distinctive name.