How did they build the Pantheon?
I have to assume that this question is less about Roman architectural techniques and materials generally and more about the thing that’s super distinctive about the Pantheon, so that’s what I’m gonna talk about.
The Pantheon is a big Roman temple in the heart of the city of Rome. The name Pantheon (or Pantheum) is not on the building itself anywhere, but it’s mentioned in ancient Latin texts. It’s Greek for “[Temple to] All the Gods” and seems to have been a nickname given to the building because it housed cult statues of multiple patron deities of the imperial family, including Mars and Venus. The Pantheon is also known today (and for the last several hundred years) as the Church of Santa Maria della Rotonda, and that name is a big clue to the thing that’s impressive about it: the rotunda. From the front the Pantheon looks like a fairly standard Roman temple with a triangular pediment and colonnaded porch, but from the side, you see that it isn’t rectangular like a normal temple; it has a humongous round butt sticking out the back, and once you go inside, it turns out to have a massive domed ceiling that you can’t easily see from the front. We used to think that the Pantheon was originally built as a fairly ordinary rectangular temple in the reign of Augustus, the first emperor (r. 31 BC – AD 14), by his right hand man Marcus Agrippa (whose name is on the dedicatory inscription), and was subsequently rebuilt as its gloriously unique self by Hadrian (r. AD 117-138) after being destroyed in a fire; this is what I was taught when I was in high school, back in the 1840s. New research says that, in fact, the Pantheon we have today was probably built during the reign of Hadrian’s predecessor Trajan (r. AD 98-117), and Agrippa’s original Pantheon probably also had a dome.
So… whence the dome?
The Pantheon had the largest domed ceiling of any building in the world when it was first constructed, and I believe this was still the case until the completion of St. Paul’s Cathedral in London at the beginning of the 18th century (even the Hagia Sophia in Constantinople, now Istanbul, doesn’t surpass it). And basically the trick is it’s made of concrete, which 1) earlier civilisations didn’t have, and 2) the Romans made using special volcanic sediment from the Bay of Naples, resulting in something lighter and stronger than even typical modern unreinforced concrete. You don’t really “pour” Roman concrete the way we do today, so much as you layer it up with strata of broken pots and bricks and rocks and other $#!t, but once it sets you still get something that is, essentially, a single piece of concrete. The dome doesn’t have any joins, it’s all one thing. The coffers – the decorative pattern of square “holes” on the inside surface – also reduce the weight quite a bit, as does the oculus, the big hole in the middle. The other trick is the dome isn’t as high as it looks from the inside. It’s really wide, but if you see it from above it’s obviously much flatter than if you see it from beneath. This is because what looks from inside like the base of the dome is actually the inside of the wall; the actual dome starts halfway up. The inside of the Pantheon is a sphere, thought to have been designed as a representation of the celestial sphere of the heavens, but its outside is a high drum. Because of the dome’s true shape and the way it was built, it requires a lot less buttressing than you’d think, and there are also a lot of arches and vaults hidden inside the walls that distribute the weight efficiently. And people ordinarily wouldn’t see it from the outside; today you can walk around the Pantheon and look at it from all directions, but it was originally part of a temple precinct that was set up so you would only approach the building from directly in front; you’d only see the outside from the one direction the architects had in mind. It’s a crafty building!