An arch turned round
A dome is what you get if you take an arch and rotate it around its centre. Like an arch, it works in compression: its weight runs down the curved shell towards the base. And like an arch, it pushes outwards as it does so, only now the push comes from every direction at once, all the way round. A dome is a ring of arches leaning on each other. That is why the earliest monumental domes, the Pantheon first among them, sit on very heavy walls.
Small domes are ancient: round huts and tombs in the Near East and around the Mediterranean were roofed this way long before Rome. What the Romans introduced was the large dome, and they could do it because of their concrete, which let them cast a curved shell as one mass instead of fitting thousands of cut stones.
Inside the Pantheon
The building you see today was rebuilt under the emperor Hadrian, in the first years of his reign (118-125 CE), on the site of an earlier temple built by Marcus Agrippa in 27-25 BCE. Behind the porch of Corinthian columns stands a cylinder of brick-faced concrete, the rotunda, with walls more than 6 metres thick, and on it rests the dome. Its span is 43.30 metres, and the height from the floor to the top is exactly the same, so a sphere 43.30 metres wide would fit inside the room and just touch the floor.
How do you keep a shell that wide from crushing its own walls? The Roman builders made it lighter as it rose. The concrete of the foundations and lower walls uses heavy basalt as its aggregate; higher up comes brick and tufa; near the centre of the dome, pumice, a volcanic glass so full of air that it floats on water. The upper part of the rotunda wall, seen from outside, rises above the springing of the dome inside, so it presses on the base of the shell like a collar. Brick arches and piers buried inside the walls carry the loads down to the foundations. The exact method of construction has never been worked out, but the quality of the mortar and the careful grading of the stone are known to have played their part.
Why leave a hole in the roof?
At the top of the dome is an opening about 8 metres wide, the oculus (Latin for eye). It is the only window. Light enters as a single shaft that moves across the coffered ceiling and the marble walls during the day. The coffers, the square sunken panels of the ceiling, are thought to have been cut under the emperor Septimius Severus, early in the third century, and were decorated with bronze rosettes.
The doors are a record of their own: the bronze double doors under the porch, 7 metres high, are the earliest known large examples of their kind.
Not the only Roman dome
Domes appear across Roman building, in baths as well as temples. At Baiae, on the Bay of Naples, the so-called Temple of Mercury is a round thermal pool under a dome; early travellers mistook it for a temple, and the name stuck. The Pantheon is the largest and the best preserved, and in 609 it was consecrated as a church, Santa Maria ad Martyres, which it still is.
Where to see it
The Pantheon is in the centre of Rome, on Piazza della Rotonda. According to the Ministry of Culture’s page (checked in October 2026), it opens every day from 9:00 to 19:00, last entry 18:30, and the full ticket costs €7 (€2 for ages 18-25, free under 18). Check before you go, because prices and hours change. The Terme di Baia near Naples are open Tuesday to Sunday with compulsory booking. The Rome guide suggests a route through the rest of the city.
Sources
- Ministero della Cultura, “Pantheon” (dates, dimensions, church in 609, hours and tickets).
- Encyclopaedia Britannica, “Pantheon”; “Dome”; “Pumice”.
- Ministero della Cultura, “Parco archeologico dei Campi Flegrei: Parco archeologico delle Terme di Baia”.
