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The original soil where the tombs were excavated is called tufa, a volcanic origin. It is easy to excavate the tomb because tufa is soft and when it has contact with the air became solid and strong. 

The workers who were working underground were called fossores, diggers of the tomb. They were professional people working underground directed and administered by the hierarchy of the Church, usually by a Deacon. The Church paid them, because it was part of Church's cooperation to help the poor people. It is part of the ministry of the Church. There were three types of ministry of the Church: The ministry of the word of God, ministry of Liturgy and ministry of Charity.

All the remains in the second floor were transferred to third and fourth floor in the 20th century before it was reopen again to the public by Pope Pius XI (1930).