Caius was a priest in Rome, in the third century. He wrote that if one comes to Rome, one can visit the shrines at the tombs of St. Peter and St. Paul. These tombs had been known and visited since the apostle's deaths, and are known to this day - they are in the same place where Caius knew them. The tomb of St. Paul is directly under the main altar of the Basilica of St. Paul Outside the Walls, which was built on the site of the first memorial chapel, which was itself built on the site of the original shrine. The tomb of St. Peter is directly under the main altar of St. Peter's Basilica in the Vatican City. The present St. Peter's Basilica, built in the renaissance, was built on the same site as the original St. Peter's, which was commissioned by the emperor Constantine in the fourth century. The tomb has always been directly under the altar.
Links
To read the fragments that we have from Caius: https://www.catholicculture.org/culture/library/fathers/view.cfm?recnum=1724&repos=8&subrepos=0&searchid=2464384
To listen to Mike Aquilina's episode on Hippolytus, contemporary of Caius and antagonist of Pope St. Zephyrinus: https://www.catholicculture.org/commentary/ep-17the-long-strange-trip-hippolytus-rome/
For more on the third century context in the history of the early Church, see the book: Reading the Church Fathers: A History of the Early Church and the Development of Doctrine: https://sophiainstitute.com/product/reading-the-church-fathers/
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Theme Music: Gaudeamus (Introit for the Feast of All Saints), sung by Jeff Ostrowski. Courtesy of Corpus Christi Watershed: https://www.ccwatershed.org/