When the barbarians (Goths and Longobards) invaded Italy and came down to Rome,they systematically destroyed a lot of monuments and sacked many places, including the catacombs. Powerless in the face of such repeated pillages, towards the end of the eighth century and the beginning of the ninth, the Popes ordered to remove the relics of the martyrs and of the saints to the city churches, for security reasons.
When the transfer of the relics was completed, the catacombs were no longer visited; on the contrary, they were totally abandoned, with the exception of Saint Sebastian, Saint Lawrence and of Saint Pancratius. In the course of time, landslides and vegetation obstructed and hide the entrances to the other catacombs, so that the very traces of their existence were lost. During the late Middle Ages they didn't even know where they were.
The exploration and scientific study of the catacombs started, centuries later, with Antonio Bosio (1575-1629), nicknamed the "Columbus of subterranean Rome". In the last century the systematic exploration of the catacombs, and in particular of those of Saint Callixtus, was carried out by Giovanni Battista de Rossi (1822-1894), who is considered the father and founder of Christian Archaeology.
In 1930 the Holy See, as the owner of the Catacombs, entrusted the catacombs of St. Callixtus to the care of the religious Congregation of the Salesians of Don Bosco.