The Pantheon: Building the Impossible Masterpiece

Happy Friday folks,

The Pantheon is an architectural masterpiece, no doubt.

Built nearly two thousand years ago, it still contains the largest unreinforced concrete dome in the world — that alone is remarkable.

But even more astonishing is the fact that it has survived intact, in such good condition for so long and that’s no accident… it’s all to do with self-healing Roman concrete…

The Pantheon we see today is not the same structure that was built 2,000 years ago. The original Pantheon was built by Marcus Agrippa, the right-hand man of Emperor Augustus, sometime around 27 BC…

Originally, it was a temple dedicated to 12 Roman gods, the name Pantheon meaning “of all the gods” in Greek. We know very little about what this first Pantheon looked like because it did not last that long. Around 80 AD, during the reign of Titus, this first (wooden) Pantheon was destroyed.

It was then rebuilt by Domitian, but that too was struck by lightning and burned down again, just 30 years later, in 110 AD.

The Beginning of a Legacy

But the version of the Pantheon that stands today was built under Emperor Hadrian, around 125 AD. Instead of taking credit for it, Hadrian chose to inscribe the name of Agrippa on the front.

The words: “M·AGRIPPA·L·F·COS·TERTIVM·FECIT” are still clearly visible on the facade, meaning “Marcus Agrippa, son of Lucius, made [this building] when consul for the third time.”

We don’t know exactly why Hadrian did this, but some say it was a sign of humility; others believe that it was simply to honor the original that was dedicated to Romulus, the mythical founder of Rome.

But in 202 AD, Septimius Severus and Caracalla made sure their restoration was known. You can barely see it, but beneath Agrippa’s bold inscription, they engraved:

^{“IMP[ERATOR] CAES[AR] L[UCIUS] SEPTIMIUS SEVERUS … ET IMP[ERATOR] M[ARCUS] AURELIUS ANTONINUS… PANTHEUM VETUSTATE CORRUPTUM CM OMNI CULTU RESTITUERUNT”}

For centuries that followed, the Pantheon remained an active pagan temple and also served as a courthouse. However, as the early Middle Ages approached, the city had fallen into decline.

In 609 AD, the Byzantine Emperor Phocas gifted the building to Pope Boniface IV, who consecrated it as the Church of St. Mary and the Martyrs. This act saved it from the fate of so many other Roman buildings, which were left to decay or were simply stripped for materials.

While temples to Jupiter and Mars were dismantled, their marble columns hauled away, the Pantheon remained intact because it had become a church.

How to Build for Eternity

But there is another reason for the longevity… concrete.

Roman concrete was so advanced for its time that we are still discovering new elements about it that are mind-blowing. Unlike modern concrete, which tends to crack and degrade over time, Roman concrete has a unique ability to repair itself. This was because it contained lime clasts — burnt small, reactive bits of lime embedded within the mix of ash.

When cracks formed, water would seep in and react with these lime clasts, producing calcium carbonate, which filled the gaps and prevented further deterioration.

Arguably, this self-healing property in itself is the most crucial aspect of why the Pantheon and its dome are standing for so long. The dome spans an astonishing 43.3 meters (142 feet) in diameter, which makes it the largest unreinforced concrete dome in the world (even after a timespan of 2,000 years).

And yet, it does not collapse under its own weight.

The secret lies in the way the Romans carefully reduced the density of the concrete as the dome rose. At its base, the concrete is mixed with heavy travertine, then terracotta tiles, while at the top, it contains lightweight pumice, a volcanic stone that keeps the upper sections from exerting too much force on the structure below.

Then, there is the gaping hole at the center of the dome — the oculus. It wasn’t put there for purely aesthetical reasons. In fact, it helps to reduce and distribute the weight at the weakest point of the dome, acting as a compression ring.

Something you often wonder about is what happens to the water when it rains. What you can’t see is that the floor is slightly slanted, allowing the rainwater to run off into the holes in the floor, draining properly. Everything in the Pantheon has a solution.

Making the Impossible

What about the gigantic portico of the Pantheon?

There are 16 solid granite columns, each measuring 39 ft (12 meters) tall and 5 ft (1.5 meters) in diameter and weighing 60 tons. But what is even more remarkable is that each one had to be quarried and then imported all the way from Egypt.

And no two columns are the same — one might be slightly more curved, another more rounded, or another fat.

Also, notice how the top of the columns is narrower than the bottom. This was on purpose: following a method called entasis. Else, the visual distortion of straight columns that would appear wider at the top, to the human eye.

The amount of thinking going into the Pantheon doesn’t end there… Symbolism was everywhere.

Its dome has five rows of 28 square coffers, perhaps quite an unusual way to separate a dome.

But 28 is one of the four perfect numbers that were known in ancient times (6, 496, and 8,128 were the others). A perfect number was one whose sum of factors equals the number itself (1 + 2 + 4 + 7 + 14 = 28).

And I’ll leave you with this…

The Pantheon’s design is based on pure geometric perfection.

The distance from the floor to the oculus and the diameter of the interior rotunda are both exactly 142 feet (43.3 meters).

This means you could fit an enormous imaginary sphere inside, and it would touch the floor, walls, and ceiling — perfectly.

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Until Next Time,

World Scholar