Pantheon, Rome

Thirteen hundred years

The Pantheon's concrete dome, 43.3 metres in diameter and 43.3 metres tall from floor to oculus, was the largest unreinforced concrete dome in the world from its completion around AD 125 until Filippo Brunelleschi raised the cupola of Florence Cathedral in 1436.

Exactly thirteen hundred and eleven years.

No other Roman dome of remotely comparable size survives intact. No European dome of comparable size was attempted at all during the intervening millennium.

Concrete the Romans had and we lost

The Pantheon's concrete is unlike anything used today.

The Romans mixed volcanic ash (pozzolana) from the slopes of Vesuvius with quicklime, fresh water, and rough aggregate — but they varied the aggregate by height. At the foundation level, heavy basalt provides structural mass. Up the walls, the aggregate changes to lighter travertine, then brick, then the lightest of all — pumice from the volcanic regions — for the upper dome.

The dome is also coffered, with five concentric rings of recessed square panels that reduce the dead weight by perhaps 30 per cent. The oculus at the top — the 9-metre open eye — both removes the heaviest part of the structure and provides the only direct light.

Hadrian's modesty

The original Pantheon on this site was built by Marcus Vipsanius Agrippa, son-in-law of Augustus, around 25 BC. That building burned in AD 80 and was rebuilt, then burned again in AD 110 in a fire caused by a lightning strike.

Emperor Hadrian (reigned 117–138) built the current structure between AD 118 and 128.

But Hadrian deliberately preserved Agrippa's original dedication inscription on the porch:

M·AGRIPPA·L·F·COS·TERTIVM·FECIT

"Marcus Agrippa, son of Lucius, made this when consul for the third time."

Why is debated. Modesty (Hadrian generally avoided self- promotion). Respect for his predecessor. Or simply that the original temple of all gods (panthei) should bear the name of its founder.

The deception worked for fifteen centuries — until the 1890s, when stamped tiles inside the building proved the construction date.

A pagan temple, then a church

The Roman Pantheon survived where almost every other pagan temple in Rome was demolished or quarried for material because in AD 609 Pope Boniface IV consecrated it as the Christian church of Santa Maria ad Martyres. The conversion preserved the building intact.

The bronze ceiling of the porch was stripped in 1625 by Pope Urban VIII — of the Barberini family — and used for Bernini's baldachin in St. Peter's. The Roman rhyme that survived: quod non fecerunt barbari, fecerunt Barberini. "What the barbarians did not do, the Barberini did."

Otherwise the building stands today essentially as Hadrian's architects left it.