The purpose of our study is to research Peter’s memory ad catacumbas. According to the Depositio Martyrum ‒ a document of the late Emperor Constantine ‒, there was no memory of the first St Peter’s Basilica on the Vatican Hill. We start with a critical analysis on the Roman Basilicae attributed to Emperor Constantine in the Liber Pontificalis, then we deepen the search of Peter’s memory in the catacombs of the SS Marcellino and Pietro (ad Duos Lauros), also known as Tor Pignattara. Indeed, the Basilica and Mausoleum built in this cemeterial area are the only buildings attributable, with certainty, to Emperor Constantine, who wished to be buried in the Mausoleum, close to an apostle. Besides some striking archeological finds on Peter’s memory already discovered near a particular cubicle in these catacombs, a geometrical and mathematical study of the unusual architectonic characteristics of the Basilica and Mausoleum of Tor Pignattara shows that the buildings were part of a single architectonic plan, very likely designed for coding data useful to locate Peter’s burial site unambiguously, in the area of the cubicle mentioned.