The safety of the catacombs was not due to the fact that their existence was known only to the proselytes of Christ. The magistrates possessed a thorough knowledge of their location, number, and extent; and we have evidence of raids and descents by the police on extraordinary occasions, as, for instance, during the persecutions of Valerian and Diocletian. The ordinary entrances to the catacombs, which were known to the police, were sometimes walled up or otherwise concealed, and new secret outlets opened through abandoned pozzolana quarries (_arenariæ_). Some of these outlets have been discovered, or are to be seen, in the cemeteries of Agnes, Thrason, Callixtus, and Castulus. In May, 1867, while excavating on the southern boundary line of the Cemetery of Callixtus, de Rossi found himself suddenly confronted with sandpits, the galleries of which came in contact with those of the cemetery several times. The passage from one to the other had been most ingeniously disguised by the _fossores_, as those who dug the catacombs were called.[149] The defence of these cemeteries in troubled times must have caused great anxiety to the Church. Tertullian tells how the population of Carthage, excited against the Christians, sought to obtain from Hilarianus, governor of Africa, the destruction of their graves. "Let them have no burial-ground!" (_areæ eorum non sint_) was the rallying cry of the mob. The catacombs are unfit for men to live in, or to stay in even for a few days. The tradition that Antonio Bosio spent seventy or eighty consecutive hours in their depths is unfounded. When we hear of Popes, priests, or their followers seeking refuge in catacombs, we must understand that they repaired to the buildings connected with them, such as the lodgings of the keepers, undertakers, and local clergymen. Pope Boniface I., when molested by Symmachus and Eulalius, found shelter in the house connected with the Cemetery of Maximus on the Via Salaria. The crypts themselves were sought as a refuge only in case of extreme emergency. Thus Barbatianus, a priest from Antiochia, concealed himself in the Catacombs of Callixtus to escape the wrath of Galla Placidia. Many attempts have been made to estimate the extent of our catacombs, the length of their galleries, and the number of tombs which they contain. Michele Stefano de Rossi, brother of the archæologist, gives the following results for the belt of catacombs within three miles of the gates of Servius:[150]-- (A) Surface of tufa beds, capable of being excavated into catacombs, 67,000,000 square feet. (B) Surface actually excavated into catacombs, from one to four stories deep, 22,500,000 square feet,--more than a square mile. (C) Aggregate length of galleries, calculated on the average construction of six different catacombs, 866 kilometres, equal to 587 geographical miles.