They were the official cemetery of the Church of Rome in the 3rd century AD. Around half a million Christians were buried here, among them many martyrs and 16 popes. They are named after the deacon St. Callixtus who, at the beginning of the 3rd century AD, was assigned by Pope Zephyrinus to the administration of the cemetery. The Callistian complex is the property of the A.P.S.A. Administration of the Patrimony of the Apostolic See. Governance, custody, and preservation are entrusted to the P.C.A.S. Pontifical Commission of Sacred Archaeology, a body of the Holy See. Since 1930 the reception of pilgrims and guided tours of the catacombs open to the public have been entrusted to the Salesians of Don Bosco.