Who actually built the Pantheon

What the bricks say

Roman bricks carry stamps, and stamps carry dates.

Traditional scholarship gave the whole thing to Hadrian, between 118 and 125 CE. Then re-examination of the brickstamps — work by Wolf-Dieter Heilmeyer, taken further by Lise Hetland in 2007 — found that the great majority of stamps in the deep foundations and in the core of the rotunda wall date to the late Trajanic period, around 113 to 115 CE.

That places the site clearance, the foundations and the structural design under Trajan, starting soon after the fire of 110 CE. Trajan died in 117. Hadrian carried out the vaulting and the interior and dedicated the finished building around 125 to 128 CE.

So: Agrippa's name, Trajan's foundations, Hadrian's dome. The research rates the brickstamp evidence high confidence, and it is the sort of finding that quietly rewrites a caption.

Yes, it rains inside

The most persistent myth about this building is that rising warm air from the crowd stops rain coming through the oculus.

It does not. There is no glass in the opening, rain falls straight through it, and thermal updraughts from bodies or candles are nowhere near strong enough to stop precipitation. The reason the floor is not a pond is engineering: the marble is laid with a slight convex camber, and there are 22 drainage holes cut into it, connected to Roman drains that still work.

Look down as well as up.

A room built as a sphere

The interior height from floor to oculus is 43.3 metres, and the interior diameter of the rotunda is also 43.3 metres. A perfect sphere of that diameter would fit inside, touching the floor, the walls and the apex at once.

The dome is coffered in 28 radial rows, in five horizontal rings, 140 coffers in all. Twenty-eight was a "perfect number" in Pythagorean mathematics — it equals the sum of its divisors — and the choice is unlikely to be accidental in a building this deliberate.

The dome was meant to read as the vault of heaven, with the oculus as the eye of it. The seven large niches in the drum wall have long been associated with the seven visible heavenly bodies — Sun, Moon, Mars, Venus, Jupiter, Saturn, Mercury — though whether Pantheon meant "all the gods" or specifically the planetary gods is disputed, and the research holds that at medium confidence.

And there is no hidden reinforcement. It is still the largest unreinforced concrete dome in the world; ground-penetrating radar and core drilling have looked, and there are no iron chains in it. It stands up by grading the concrete lighter towards the top — the shell thins from about 6.4 metres at the base to roughly 1.2 metres at the oculus ring — and by relieving arches buried in the drum.

Why the front doesn't quite fit

Stand back and look at where the porch meets the rotunda. The pediment sits awkwardly low against the block behind it, and there is a visible step.

The likeliest explanation is that the portico was designed for 50-foot granite columns and got 40-foot ones — the monoliths that were ordered did not arrive from Egypt, substitutes were used, and the roofline never recovered. The research rates that medium confidence. It is a two-thousand-year-old procurement problem, frozen in stone.

Everything that was stripped off it

The Pantheon survives because it was converted, not because it was left alone.

• 663 CE — the Byzantine emperor Constans II removed the gilded bronze roof tiles.
• 800s–1100s — Tiber flooding damaged the floor marble, and medieval shops and huts were built hard against the outside of the drum.
• 1626 — Urban VIII had the ancient bronze ceiling trusses taken out of the portico, around 150 tons of metal. The line you will hear is that it went into Bernini's baldachin in St Peter's. Papal records show most of it was cast into 80 cannons for Castel Sant'Angelo, with only a fraction going to Bernini.
• 1660s–1883 — two Baroque bell towers from Bernini's workshop stood on the front. Romans called them the orecchie d'asino, the donkey ears, and after unification they were pulled down.

What kept it standing

In 609 CE Pope Boniface IV consecrated the building as a church, and 28 cartloads of martyrs' bones were brought up from the catacombs and reburied here. That conversion is the single reason the Pantheon was maintained while the rest of imperial Rome was quarried.

The medieval legend attached to that day — that the temple was full of demons who fled through the roof as the cross went up, punching the oculus open — is rated low confidence in the research, which is a polite way of saying it is a story. The oculus was designed with the dome.

Raphael asked to be buried here, and is.

The petals

Fifty days after Easter, at the Pentecost mass, Rome's firefighters climb onto the outside of the dome and drop thousands of red rose petals through the oculus.

They fall the full 43 metres through the shaft of light onto the people below. It is a living tradition in a two-thousand-year-old room, and it uses the building exactly as it was designed to be used.

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