The catacombs are of Christian origin, built at Rome in the middle stratum of tufa, from which no building material was quarried. They are reached by a stairway which leads below to a depth of from 33 to 49 feet. There is a labyrinth of narrow galleries, sometimes found in three or four stories. The burial chambers are hollowed out of the rock and there are tiers of graves along the gallery walls. The graves were marked by slabs. Caemeterium, place of rest, was the early name. Ventilation was given by air shafts, or luminaria.
The catacombs originated in connection with the tombs which belonged to the early converts from paganism. They imitated the cemeteries which the Jews living at Rome had laid out in the manner of the rock-graves of Palestine. According to Roman law places of burial were sacred. After Constantine cemeteries were laid out above ground, but burial in the catacombs was not discontinued until 410. Later the remains of the martyrs were transferred to the churches and from the 12th to the 16th century the catacombs were forgotten.
The labors of Bosio and De Rossi inspired a new interest in Christian archaeology. The inscriptions of the catacombs have yielded more information than any other source concerning the first Christian centuries. The spaces between graves, the arched niches, and the walls of the chambers are adorned with paintings. The souls of the dead are represented as Orantes (praying) in the bliss of heaven. Episodes from the Bible are chosen which depict the belief in a future life. The Madonna with the Divine Child is found, and the Saviour enthroned in glory. Early sculpture is represented by the sarcophagi used by the converts from paganism. Towards the end of the 3rd century they were ornamented with the same biblical scenes but with greater variety of design, and the life of Christ is represented. Catacombs have also been, discovered in Naples, Sicily, Sardinia, Malta, and northern Africa. Pope Pius XI had the Roman catacombs illuminated by electricity.