The Catacombs are subterraneous chambers and passages usually formed in the rock, which is soft and easily excavated, and are to be found in almost every country in which such rocks exist. In most cases, probably, they originated in mere quarries, which afterwards came to be used either as places of sepulchre for the dead, or as hiding-places for the persecuted living. The most celebrated Catacombs in existence are those on the Via Appia, at a short distance from Rome. To these dreary crypts the early Christians were in the habit of retiring, in order to celebrate Divine worship in times of persecution, and in them were buried many of the saints, the early Popes, and martyrs. They consist of long narrow galleries, usually about eight feet high and five wide, which twist and turn in all directions. The graves were constructed by hollowing out a portion of the rock, at the side of the gallery, large enough to contain the body. The entrance was then built up with stones, on which usually the letters D. M. (Deo Maximo), or [CHR], the first two letters of the Greek name of Christ, were inscribed. Though latterly devoted to purposes of Christian interment exclusively, it is believed that the Catacombs were at one time used as burying-places for Pagans also, and there are one or two which were evidently entirely devoted to the Jews. At irregular intervals, these galleries expand into wide and lofty vaulted chambers, in which the service of the Church was no doubt celebrated, and which still have the appearance of chapels. The original extent of the Catacombs is uncertain, the guides maintaining that they have a length of twenty miles, whereas about six only can now be ascertained to exist, and of these, many portions have either fallen in or become dangerous. When Rome was besieged by the Lombards in the 8th Century, several of the Catacombs were destroyed, and the Popes afterwards caused the remains of many of the saints and martyrs to be removed and buried in the churches. The Catacombs at Naples, cut into the Capo di Monte, resemble those at Rome, and evidently were used for the same purposes, being partially covered with remarkable Christian symbols. At Palermo and Syracuse, there are similar Catacombs, and they are also to be found in Greece, Asia Minor, Syria, Persia, and Egypt. At Milo, one of the Cyclades, there is a hill which is honeycombed with a labyrinth of tombs running in every direction. In these, bassorilievi and figures in terra-cotta have been found, which prove them to be long anterior to the Christian era. In Peru and other parts of South America, ancient Catacombs still exist. The Catacombs of Paris are a species of charnel-house, into which the contents of such burying-places as were found to be pestilential, and the bodies of some of the victims of the Revolution, were cast by a decree of the Government. The skulls are arranged in curious forms, and a visit to these weird galleries is one