There is no single circumstance, in relation to the catacombs, of more striking and at first sight perplexing character than their vast extent.  About twenty different catacombs are now known and are more or less open,—­and a year is now hardly likely to pass without the discovery of a new one; for the original number of underground cemeteries, as ascertained from the early authorities, was nearly, if not quite, three times this number.  It is but a very few years since the entrance to the famous catacomb of St. Callixtus, one of the most interesting of all, was found by the Cavaliere de Rossi; and it was only in the spring of 1855 that the buried church and catacomb of St. Alexander on the Nomentan Way were brought to light.  Earthquakes, floods, and neglect have obliterated the openings of many of these ancient cemeteries,—­and the hollow soil of the Campagna is full “of hidden graves, which men walk over without knowing where they are.”

Each of the twelve great highways which ran from the gates of Rome was bordered on either side, at a short distance from the city wall, by the hidden Christian cemeteries.  The only one of the catacombs of which even a partial survey has been made is that of St. Agnes, of a portion of which the Padre Marchi published a map in 1845.  “It is calculated to contain about an eighth part of that cemetery.  The greatest length of the portion thus measured is not more than seven hundred feet, and its greatest width about five hundred and fifty; nevertheless, if we measure all the streets that it contains, their united length scarcely falls short of two English miles.  This would give fifteen or sixteen miles for all the streets in the cemetery of St. Agnes."[B] Taking this as a fair average of the size of the catacombs, for some are larger and some smaller, we must assign to the streets of graves already known a total length of about three hundred miles, with a probability that the unknown ones are at least of equal length.  This conclusion appears startling, when one thinks of the close arrangement of the lines of graves along the walls of these passages.  The height of the passages varies greatly, and with it the number of graves, one above another; but the Padre Marchi, who is competent authority, estimates the average number at ten, that is, five on each side, for every seven feet,—­which would give a population of the dead, for the three hundred miles, of not less than two millions and a quarter.  No one who has visited the catacombs can believe, surprising as this number may seem, that the Padre Marchi’s calculation is an extravagant one as to the number of graves in a given space.  We have ourselves counted eleven graves, one over another, on each side of the passage, and there is no space lost between the head of one grave and the foot of another.  Everywhere there is economy of space,—­the economy of men working on a hard material, difficult to be removed, and laboring in a confined space, with the need of haste.