The catacombs of Rome are very ancient cemeterial areas digged in tufa, a kind of soft rock. The oldest ones are from the II century A.D. They were all digged outside the roman walls because during those times it was forbidden to bury people inside the city. When in 313 A.D. thanks to Costantine the christians were free to worship their god, many of them wanted to be buried in these underground cemeteries where many martyrs were placed years before. Centuries later the catacombs were abandoned and the relics of the martyrs were transferred inside many churches. For several centuries nobody knew anymore where the catacombs and their entrances were. It was just in the XIX century that they started to be explored sistematically. In the catacombs there were also beautiful cripts, like the cript of the popes in the catacombs of St. Callixtus. Some of the first popes of the christian history have been buried here.