Why does the Pantheon have a hole in the roof?

You can't see it from the square. From outside, the Pantheon is a columned porch stuck onto a great, rather plain brick drum. The surprise is inside: you look up, and instead of closing over you, the dome opens to the sky.

What is the hole in the Pantheon for?

Light. The great topographical dictionary of ancient Rome, by Platner and Ashby, describes it as a circular opening, ringed by a bronze cornice, "through which light is admitted". Encyclopaedia Britannica goes further: the vast round space is lit solely by the light that floods in through that "eye". Hence the name: oculus is simply Latin for eye.

The hole is almost nine metres across. And here's the detail geometry fans love: from the floor up to the oculus it's 43.20 metres, exactly the same as the width of the hall.

Do the maths with us. The dome is half a sphere, sitting on a cylinder as tall as the dome itself. Complete that sphere downwards and a giant ball would fit snugly inside the building, just touching the floor. That's our own calculation from those measurements, but it helps explain why the interior feels so perfectly ordered.

How does a dome with a hole in it stay up?

With a great deal of skill, and partly in ways we still don't fully understand. According to Britannica, no dome in the world was bigger until modern times, and exactly how it was built has never been fully worked out. What is clear is that its builders thought very hard about weight.

The walls of the drum are more than six metres thick, but they aren't solid. Inside, they're full of niches, voids and hidden arches that spread the load. And the concrete isn't the same from bottom to top. In the foundations and the lower walls there's basalt, a heavy stone. Higher up, brick and tufa, a volcanic stone. And towards the centre of the vault, pumice, the lightest of all. The higher you go, the lighter it gets.

And the hole? You'll often hear that it was also left to take weight off the most delicate part of the dome, right at the top. It makes sense, and it may well be true. But none of the reference works we've consulted gives it as the reason for the oculus, and we know of no ancient text that says so. What's documented is the light.

Who built the Pantheon?

Large letters across the porch frieze announce that Marcus Agrippa, consul three times over, made it. Agrippa, Augustus's closest ally, put up a first temple here in the emperor's day. It burned in AD 80, was restored and, some years later, caught fire again after a lightning strike.

The one you see is a different building. The emperor Hadrian rebuilt it in the second century, foundations and all. The proof is in the bricks: the drum has yielded numerous makers' stamps from his reign. Hadrian left his own name off, and he was very probably the one who had Agrippa's put up instead: according to an ancient biography, he dedicated every building he restored to its original founder. And to cap it all, the bronze letters you read on the frieze today are a modern replacement.

Hadrian didn't just build it, either. According to the historian Cassius Dio, he even held court in the Pantheon. Picture the emperor dealing with the business of the day under that open eye.

Why is it called the Pantheon?

Nobody knows for certain. The name may refer to the many gods honoured inside, or mean something like "most holy". Dio, writing in the third century, offered two possibilities: that the temple held images of many gods or, as he preferred to think, that its vault resembles the heavens.

Platner and Ashby's dictionary gives that second explanation no credit as the origin of the name. As a visitor's impression, though, it's hard to argue with: a dome as round as the sky, with an eye open at its centre. That last bit is our opinion.

Centuries later, the building was still turning heads. In the fourth century, the historian Ammianus Marcellinus counted it among the wonders of Rome, comparing it to a round district roofed over by a vault of splendid height.

Does it rain inside the Pantheon?

Yes. The oculus is a hole open to the sky, with no glass and no cover, so when it rains, the rain comes in. As for what happens next, you'll hear all sorts of explanations: that warm air rising from inside breaks up the drops so hardly anything gets wet, that the floor slopes so the water runs off, even the exact number of little drainage holes. We haven't found a reliable source that explains it properly, so we'd rather not give you figures.

What has changed is the roof. The outside of the dome was covered in bronze tiles until, in the seventh century, a Byzantine emperor had them stripped off. Later, a pope had it covered in lead. And inside, the coffered ceiling was originally gilded.

That it's come down to us in one piece has a lot to do with another date: in 609, Pope Boniface IV consecrated it as a church. That this is what saved it is what people usually say. It's plausible, but it's an interpretation.

Fact or legend.

• Fact: the oculus is the opening that lets the light in, and the interior is lit only through it.
• Fact: from the floor to the oculus is 43.20 metres, the same as the width of the hall. The sphere that would fit inside is our own calculation from those measurements.
• Fact: the dome's concrete gets lighter the higher it goes, with pumice towards the centre of the vault.
• Widely repeated, but not backed by the reference works we've read: that the hole was left to take weight off the dome.
• Unconfirmed: that hardly any rain gets in thanks to rising air, and the number of drains in the floor.
• An ancient opinion: Cassius Dio thought the name came from the vault resembling the heavens; Platner and Ashby's dictionary rules it out as the origin of the name.
• Fact: the present building is Hadrian's work, even though the frieze says Agrippa.

Where to hear it

The Pantheon has three stops in a row on "The Pantheon, the Minerva and Piazza Colonna", Podtour's free route in Rome. "The wrong name" tells the story of the borrowed name; "The building that survived" explains how it made it through; and "Raphael wanted to stay here" tells you about the oculus and that exact measurement, alongside the story of the painter who wanted to be buried beneath it.

All three are heard from the square; the oculus is waiting for you if you go inside. The whole route is eleven stories in just over a kilometre, from the Column of Marcus Aurelius to Borromini's spiral.

If you're planning your trip, our Rome guide has the practical side covered.

Further reading

• Samuel Ball Platner and Thomas Ashby, "Pantheon", A Topographical Dictionary of Ancient Rome, Oxford University Press, 1929, pp. 382–386; LacusCurtius transcription. https://penelope.uchicago.edu/Thayer/E/Gazetteer/Places/Europe/Italy/Lazio/Roma/Rome/_Texts/PLATOP*/Pantheon.html
• Encyclopaedia Britannica, "Pantheon" (by the editors of the encyclopaedia). https://www.britannica.com/topic/Pantheon-building-Rome-Italy
• Cassius Dio, Roman History, book LIII, chapter 27, Loeb Classical Library translation on LacusCurtius, with the translator's note on the oculus. https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Cassius_Dio/53*.html
• Historia Augusta, "Life of Hadrian", chapter 19, Loeb Classical Library translation on LacusCurtius. https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Historia_Augusta/Hadrian/2*.html