Rome’s Pantheon: The Secrets of the World’s Largest Dome of Its Time

Founded more than 28 centuries ago in central Italy, 30 kilometers from a maritime outlet on the site of seven hills, Rome is considered the second cradle of Western civilization, after Athens. The most populous city in Italy today has 4,300,000 inhabitants within its urban area. Rome is also the third most visited city in the world, with more than 10 million foreign visitors every year. On the one hand, because it's the only city in the world to contain within it a state, the Vatican City State, subject to the authority of the Pope, that attracts Christians of all nations. But also because of the city's own archaeological heritage, grandiose. Here, there are so many ancient monuments, all of them both artistic and technical achievements. How, one wonders, did the Romans accomplish such feats? They achieved the impossible with this unique monument, the most complete and best preserved of all antiquity, the Pantheon. Its architecture is extraordinary. No such thing had ever been seen before in antiquity. A gigantic environment, in which space and light dominated, with this circular shape, completely original, which was then copied in so many Renaissance buildings. Situated on the Piazza della Rotonda, in the historic heart of Rome, the Pantheon reflects a desire to welcome all the religions of the Empire, gathering all their gods in one great temple. It is topped by a dome 43 meters in diameter and weighing 4,535 tons, the largest in the entire Roman Empire. It is destined to be a model of architectural perfection. The Pantheon has been standing for more than 2,000 years. It all starts with a novel plan. The temple, in two parts, reconciles two types of construction, two different forms, one rectangular and the other circular. This has never been seen before. What's striking about the Pantheon is that it's an architectural feat, created to provoke surprise. So we look up and we see a temple, but we barely see the dome behind it, almost not at all. We're already struck by the monumentality of the columns, which are monolithic columns made of a single block. 11 meters for each barrel, that's 42 tons or so. Each column is the height of a four-story building and weighs as much as an A320 Airbus. So they had to think about how to transport these huge long pieces. They needed ships that could transport them from Egypt to Ostia. We know from Egyptian hieroglyphics and other ancient texts that the Romans could build ships capable of carrying loads of over 100 tons. And from Ostia, the blocks headed upriver to the port of Rome. Once mounted on a wooden sled, the column was towed along. It must have taken 10 teams of 20 oxen to haul the column to the foot of the Pantheon. When we look at things like these columns, and we're amazed by the strength of them, by the fact that the building stands here, you know, all these years, it's natural to think, how did the Romans calculate this? How did they do that? And, you know, we think of modern engineering. Now, first of all, Roman architects and engineers weren't separate people. They were one and the same people. And the next thing we have to bear in mind is they were very practical, empirical, and they had a lot of experience. They had decades of experience. But that is not the big surprise. Inside, one crosses through a building of white marble. And the second space is revealed. And one is under this huge dome, which is actually a complete sphere. Since the height of the lower half is equal to the radius of the dome above, 43 meters. Never has a dome of this size been built in the history of the world, in the history of mankind, until the invention of reinforced concrete. It's the largest dome in the world. And there are no pillars. This is an entirely new type of construction. Incredibly, they managed to build it without any support. You have to stand right in the center of it to see how the Roman builders had the idea to use the volume of a sphere and thus to create a whole new form. Just how did the Romans develop such a skill? By using a revolutionary new material, concrete. Concrete definitely represents a revolution in Roman constructions. First of all, in terms of its composition, Roman concrete is a mixture of lime, sand, water, of course aggregates. Basically, it's modern concrete. Now, this amazing material, Roman concrete, it has the great advantage that it can be made by semi-skilled people. What's more, the Romans discovered that when they added to this lime mixture some pozzolana, which is a type of volcanic stone, or some tile, that is, fragments of crushed tiles or bricks, there was a chemical reaction between the lime and the pozzolana, or the tile, that caused the stone to become stronger and more waterproof, the more that water was added to it. This Roman mortar completely transforms construction. Now their ideas can be realized on a large scale, quickly and well. The Romans built using two sidings of brick cladding filled with small stones or pebbles. This technique, similar to casting, will be as important to ancient builders as concrete is to those of today. Mortar would free the Romans from the Greek heritage of the straight line. And this meant they could go on to perfect the arch and the vault that will both characterize Roman architecture and become widely used in all grand constructions. The design of the Pantheon is especially innovative, particularly because of its circular form, and not only for its shape, its plan, but also, and above all, for its roof, this extraordinary dome which holds up all on its own. It symbolizes the celestial vault, home of the gods. It weighs 4,535 tons, the equivalent of 16 Airbus A380s. But how did these Romans build such spectacularly high vaults? The dome of the Pantheon must have been built in several stages. First of all, they built a framework of wood whose layout corresponded to the future segments or caissons. The second stage consisted of a thin layer of vaulting composed of ribs made of bricks and mortar. The third step was to manufacture and install a negative framework inside each caisson. Then they merely had to fill in the concrete in bulk, thus revealing the result without all its framework. The dome has 140 caissons. The cells have been deliberately offset to increase the effect of perspective on the visitor and add to the height of the temple, making it even more impressive. After hardening, the concrete poured into the box mold formed a very solid base. The fourth step was to reinforce the dome with a skeleton of brick arches. The higher it rises, the lighter is the mortar, incorporating more pozzolana, the abundant volcanic stone that's as light as pumice. Dome will be covered with concrete. It's a construction principle that allows the load borne by the outer part of the vault to overhang it and transmit the pressure back to the lateral pillars. Then they covered the building with a skin of marble or stucco veneer inside and tiles on the outside. I wouldn't say you could call much of their architecture beautiful or refined. Words like pretty, no, I mean it's solid, chunky, business-like, sensible. The daylight is used with theatrical skill by the central opening at the summit, an oculus or eye nearly nine meters in diameter. The light follows the course of the sun, giving the impression that the dome rotates to the rhythm of time, seeking perhaps to restore the divine mystery. And that's what makes the Pantheon unique. You enter the Pantheon and you see this gigantic space with this light coming from above. The building techniques that the Romans used were in large part learned from the Greeks, but with some very key differences. Greek architecture always used columns, beams and cornices. It was based on large blocks of marble. It's a kind of different way of thinking. Now the Romans have adopted a completely different principle, which is of how you take small pieces, not these large bits of marble. So you have bits of brick, fist-shaped sized bits of stone of various kinds, and you mix it together in a medium that is, if you like, the glue. And so the Pantheon is about the absolute maximum they ever got to. They never did a vault any bigger than that. I mean, I think you have to realise that the building has got quite severe cracks. I mean, it's something of a miracle it hasn't fallen down. And they were probably alarmed by these things, but it held. And they didn't go any bigger than that. You can get an idea of its longevity simply by remembering that these walls are six metres thick. Of course, in their manufacture, the walls were then to be adorned and decorated with plaster. The bricks were covered either in plaster or, in the more luxurious houses, in marble veneers. Unfortunately, all these marble veneers have disappeared, and what we are left with are the skeletons of monuments. This dome will remain the largest in the world for over a thousand years.