The Pantheon - Under the Dome

Some experts believe that this is one of the most important buildings the world has ever seen. Roman technology transformed lime mortar into strong, durable concrete. This revolutionized architecture. But having created this new concrete, the Romans then took it to new levels of ingenuity, unsurpassed even to this day. One example of such before-their-time skills can be found in the back streets of Rome. Architect Mark Wilson-Jones believes it to be one of the most important buildings ever built. This is the Pantheon, one of the world's most amazing monuments. Right. And there we are. It's a spectacle. Built in 118 AD, this is a temple to gods no one now worships. But it still inspires awe and wonder. The first time I saw it, it's just like it is now. You're uplifted. You feel a kind of joy at being in the presence of such an extraordinary thing. When Emperor Hadrian's architects begin designing a new temple to the 12 main gods of Roman religion, they have to come up with an impressive building. They decide upon a vast dome, 142 feet in diameter. By Roman standards, that might not sound big, but in the 2nd century, it's a massive undertaking. A solid dome, made from uniform Roman concrete, would be too heavy to be self-supporting. They need a material light enough and strong enough to span a huge gap. A large dome constantly pushes outward toward its base. So the engineers build 20-foot thick base walls to support and buttress the dome and stabilize the structure. Next is the dome itself. A team of carpenters construct a wooden framework. Masons then covered this with concrete, which they built up in rings from the base. But they risk the structure collapsing once the wooden scaffolding is removed, if they use normal concrete. Well, the important thing for a dome is that it's relatively weak at the top and you want to lighten it. Their technological skills give the Roman masons an ingenious solution. They use lighter and lighter stones within the concrete to reduce the load as the layers get higher. You want to go for the lightest possible at the top and the strongest and the heaviest at the bottom. So they were carefully graded in sections from the light at the top, bottom, and heavy at the bottom. For strength, at the base, they mixed in large lumps of heavy basalt rock. In the middle, knuckle-sized pieces of stone. At the top, they use pumice, a volcanic stone that floats in water. Finally, at the apex, they leave a 30-foot hole called the oculus. This avoids a heavy load at the dome's weakest point and also allows light into the Pantheon's interior. The engineering of the building is incredibly precise. In fact, a sphere based on the curve of the dome would fit almost exactly inside the square building. The Pantheon remains one of the greatest buildings ever constructed. It's not until 1,300 years later that anyone builds a dome as big as this, with the construction of the Duomo in Florence in 1420. Even nearly 2,000 years later, the Pantheon is still the largest unreinforced spherical concrete dome ever built. Its design still influences architecture across the world. Spot the difference. One of these buildings is 2,000 years old. The other is the New York State Supreme Court. Rome's Pantheon is the one on the left.