In December 1939, workmen preparing a tomb for Pius XI in the grottoes broke through into a brick vault nobody knew was there. What followed was a decade of excavation, kept quiet at the time, which went down through a Roman cemetery to a second-century shrine and a plain first-century grave — directly beneath the high altar, about two storeys down.
The ground was a bad neighbourhood
The Vatican side of the Tiber was outside the sacred boundary of ancient Rome and nobody wanted it. The soil was swampy, the slopes were soft tufa. Martial joked that the wine grown here was vinegar; Tacitus records northern legions dying in numbers in 69 AD when they had to camp in it.
Agrippina the Elder drained part of it for gardens. Around 40 AD her son Caligula started a private chariot track in the grounds, which Nero finished and enlarged — the Circus of Caligula and Nero. Alongside it, lining the Via Cornelia, was a cemetery.
This is the point. Christian tradition places Peter's execution in that circus and his burial in that cemetery, and the cemetery is a documented fact of Roman topography, not a later invention.
Constantine built on top of it, and it cost him
After the Edict of Milan, Constantine ordered a basilica over the venerated grave, begun between 319 and 326 AD.
The site was a hillside, so his engineers sliced into the slope and filled the working pagan mausoleums with thousands of tons of earth to make a level terrace. They buried a functioning cemetery to build on it — an enormous and deliberately awkward decision, taken because the position of one grave was not negotiable.
That fill is why the necropolis survived so completely. It was sealed in the fourth century and left.
What the Scavi found, in order
Pius XII authorised the excavation. Monsignor Ludwig Kaas oversaw it, the Jesuit archaeologist Antonio Ferrua directed it, and it ran from 1939 to 1949.
Under the altar they found a stack, each layer built precisely over the last:
- The Papal Altar of Clement VIII, 1594
- enclosing the Altar of Callixtus II, 1123
- built over the Altar of Gregory the Great, around 600
- built over Constantine's marble casing, around 326
- built against a red-plastered wall, the Aedicule or Tropaion of Gaius, an open-air shrine of about 160 AD
- and beneath that, a simple first-century earth grave.
Seventeen hundred years of people building over the same spot, each generation certain the last one had marked it correctly.
"Peter is here"
In 1952 the epigrapher Margherita Guarducci examined plaster taken from a wall next to the shrine, covered in overlapping Greek graffiti scratched by pilgrims.
Among the fragments she read PETROS ENI — "Peter is here."
What this establishes, and what it does not. The archaeology is solid: there is a Roman necropolis, a second-century shrine venerating a grave, and pilgrim graffiti naming Peter, all directly under the altar. That tells you Christians in about 160 AD believed Peter was buried on this spot, and that everyone since has built on that belief. It is not a forensic identification. The grave is where the veneration was; whose bones were in it is a further question the excavation could not close.
The basilica had to be replaced because it was falling over
Old St Peter's stood for more than 1,100 years and by the mid-fifteenth century it was failing.
The southern nave wall had leaned outward by over three feet, because the necropolis fill underneath it was settling unevenly. The roof timbers had rotted and the interior was black with candle smoke. Nicholas V, advised by Alberti, began planning the replacement in the 1450s.
Who designed what
The building is a committee across a century, and the credit is routinely misassigned.
- Bramante set the centralised Greek-cross plan.
- Michelangelo designed the dome — a double shell on Brunelleschi's Florence principle, an inner one to carry the load and a raised outer one for the silhouette. It is 41.47 metres across inside and 136.57 metres to the top of the cross. Della Porta and Fontana completed it.
- Carlo Maderno extended the nave and designed the facade, between 1607 and 1614 — 114.69 metres wide, 45.44 metres high, in Tivoli travertine. Not Michelangelo.
- Bernini built the Baldacchino, 28.9 metres of cast bronze weighing 63 tons, and the elliptical colonnade outside: 284 Doric columns carrying 140 statues of saints.
Four things almost everyone believes that aren't true
| Claim | Verdict | What is actually the case |
|---|---|---|
| St Peter's is the cathedral of Rome | False | The cathedral of the diocese of Rome is St John Lateran. St Peter's is a papal basilica |
| Michelangelo designed the facade | False | He designed the plan and the dome. The facade and nave are Maderno, 1607–1614 |
| The Baldacchino's bronze came off the Pantheon's dome | Partly false | It came off the Pantheon's portico beams, and ledgers show most of that metal became cannon for Castel Sant'Angelo |
| Peter is buried inside the Baldacchino | False | The grave is roughly two storeys below the high altar, in the Roman necropolis |
Two things worth looking for
The Pietà carries the only signature Michelangelo ever cut into a work — across the sash on Mary's chest — added after he overheard visitors crediting the sculpture to a Lombard rival.
And the great narrative pictures on the walls are not paintings. Almost all of them are micro-mosaics, made over centuries by the Vatican Mosaic Studio, because mosaic does not decay in candle smoke and damp the way canvas does. Stand close to one.
Hear this one standing in front of it
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Written from Wanderlight's research dossier for St. Peter's Basilica — the same one the narration is written from. Where the evidence is thin, this page says so rather than rounding the uncertainty up.