Is it true that the Vatican [we assume what is meant here is the St Peters Basilica in Rome] was founded on the grave of the apostle Peter? Was the location of his grave known?
Answer: At the foot of the hilly area known as the “vaticanum” beyond the northwestern shores of the river Tiber, and outside the antique city of Rome, the Caesars Caligula and Nero owned gardens and possibly a villa which boasted a circus. Two cross country roads, the Via Cornelia and the Via Triumphalis led away from Rome across the pons Neronis (Nero’s bridge) through this area to southern Etruria. As was common outside the town, these roads were lined with cemeteries, several parts of which were uncovered in what today is Vatican City. And inscription on a burial ground of the 2nd century of the necropolis, which was partly excavated in 1940-49 beneath St Peter’s, states that this grave was erected “iuxta circum Neronis” (next to Nero’s circus). The location of this circus is known due to excavations over the last decades, and also because of the original site of the obelisk immediately south of today’s Basilica, which was moved from there to St Peter’s Square in 1587: The circus was located at the foot of the Vatican hill, approx. 600 m long, stretching from West to East, and almost parallel to today’s St Peters Basilica, which covers its northernmost end. According to the Roman historian Tacitus, it appears that numerous persecuted Christians were executed here between the years 64-67. The mention of Peter as the Roman martyr in “The Letter of Clemens to the Corinthians” (1 Clem. 1, 5-6) and in other sources, provide evidence for the oral tradition of Peter’s death as a martyr in Rome before the end of the first century. This therefore has a high degree of historical probability.
During the renovation of the grave of Pius XI in the grotto of St Peter’s in 1939, some antique walls were discovered. Comprehensive archeological research and discussions showed that the site that is venerated today as containing St Peter’s grave, has been venerated for this reason since the middle of the 2nd century with a shrine (aedicula) dedicated to St. Peter. Shortly after 320 this caused King Constantine to turn this memorial site into the cultic center of the Basilica constructed in the name of the apostle. In the tradition of the Christian community in Rome the constantly maintained and revered memorial site for Peter, the petrus memoria, has existed from the middle of the 2nd century until the era of Constantine.
In his comprehensive scientific essay "St Peter’s grave” in the Lexikon für Theologie und Kirche, vol. 8, Col 149-153, Hugo Brandenburg, however, concludes that doubts remain whether the original grave of the disciple had been precisely at this location underneath the memorial, and not only for archaeological reasons. In any case, the Memoria Petri, the St. Peter’s memorial, which has continuously been an effective site for veneration since the 2nd century, and Jesus tomb in the church of the Holy Sepulcher in Jerusalem are the most important memorials of Christendom. From a historic point of view, it is the most important archaeological memorial ever.