Roman architecture does'nt get much more awe-inspiring than the Pantheon. Erected in the Field of Mars (Campus Martius), just outside the city walls where the Roman army used to train, the Pantheon was the centrepiece of an elaborate monumental theme park which marked Rome’s transition from a city of brick to a city of marble.

The Pantheon’s famous dome, still the largest freestanding dome in the world, and imposing temple colonnade are now among the most recognizable icons of the Eternal City. But they mask a building with a fiendishly complicated history, which even the ancients writing 200 years after the Pantheon’s completion were debating. So what was the Pantheon for? Who built it? And most importantly, what is the story behind one of the most visited temples in the Rome?

What was the Pantheon?

Our first clue for this comes from the word Pantheon itself. The ancient Roman aristocracy was practically bilingual in Latin and Greek, and it was from the latter their name from this temple derived. For the Greek word for “all” was pan and the Greek for “gods” was theos. Hence the name and purpose of the Pantheon: as a temple built in honor of all the Roman gods.

Who built the Pantheon?

A straightforward question with a far from straightforward answer. The original Pantheon was built around 25 BC in the wake of the first emperor Augustus’ victory over his famous rivals Mark Antony and Cleopatra at the Battle of Actium in 31 BC. Not satisfied merely with winning, Augustus wanted to advertise his victory. So he financed an elaborate building project throughout the city paradoxically celebrating both the peace he’d brought and the war he’d fought to secure it.

Entrusted with carrying out this building project was Marcus Agrippa, the emperor’s right-hand man, contemporary, and son-in-law through his marriage to Augustus’ daughter Julia. We don’t know much about what Agrippa’s Pantheon looked like. What we do know is that he hired some of the ancient world’s finest artists, like Diogenes of Athens, to decorate it. And inside stood a statue of Venus whose earrings were made from Cleopatra’s pearls.

Agrippa built several other structures in the Campus Martius, including some public baths (the Baths of Agrippa) and the Basilica of Neptune, which housed artefacts from Augustus’ naval victory at Actium. In fact, look behind the Pantheon today and you can still see the remains of this Basilica of Neptune.

We know this much not just from ancient writers but also from the inscription on the Pantheon’s front. It reads:

M.AGRIPPA.L.F.COS.TERTIUM. FECIT

(Marcus Agrippa, the son of Lucius, built this during his third consulship.)

But the inscription is misleading. Agrippa’s Pantheon was destroyed in 80 AD in one of Rome’s many great fires. The emperor Domitian (whose father Vespasian built the Colosseum) had it rebuilt, just as he did many other ancient monuments you see in the Roman Forum and historic center today.

Then, in 110 AD, a little over 100 years after the completion of Agrippa’s original Pantheon, the Pantheon burned down again. Everything was reduced to cinders. Everything, that is, apart from Agrippa’s original inscription. It was left to the incumbent emperor Hadrian to rebuild the enormous structure. And Hadrian, being the modest man that he was, decided to incorporate the original inscription, which is the one you see before you today.

What’s so special about the Pantheon Dome?

We know almost nothing about Agrippa’s original Pantheon. It could have had a dome (the word “Pantheon” meaning “Dome to the Heavens”), but we can’t say for sure. We do know lots, however, about Hadrian’s Pantheon. And one of its most intriguing features is the enormous freestanding dome.

Weighing 4,535 tons of Roman concrete, the Pantheon’s dome is an architectural marvel. It’s the world’s largest unreinforced concrete dome, supported by eight barrel vaults. Within the structure are arches to take some of the stress of the weight, but these are skilfully hidden behind marble facing.

The Pantheon’s most famous feature is the hole in the center of its domed roof. Admittedly this causes problems when it rains or even snows. But on a beautiful Italian day, standing on the original floor of a 2,000 year old structure and gazing up at the streaming sunlight from the same spot the emperor Hadrian would have held assemblies truly is magical.

What happened to the Pantheon after the sack of Rome?

For much of the Roman Empire the Pantheon functioned as a pagan temple. Then in 609 AD, the Byzantine emperor Phocas gifted the Pantheon to Pope Boniface IV, who duly converted the pagan temple into a Christian basilica called Santa Maria ad Martyrs. Ironically, being converted into a church saved this pagan monument. Rome’s ancient and medieval population were constantly recycling building materials from ancient structures. Fearing for their souls, however, made them far less inclined to steal from churches.

The Pantheon didn’t escape completely. In 663, the emperor Constans II stripped the Pantheon of its bronze tiles, which he shipped back to Constantinople. But the Pantheon fared better than most monuments of the Roman Empire, as a tour of the Roman Forum makes perfectly clear.

What’s inside the Pantheon?

The Pantheon contains the bodies of some of Italy’s Renaissance greats. Raphael, whose Raphael Rooms are one of the Vatican Museums’ most prized attractions, rests eternally within. So too does Baldassare Peruzzi, whose enviable legacy of architecture includes the Villa Farnesina. In 1870, the Pantheon became home to the remains of Italy’s royal family. Two kings rest within, Vittorio Emanuele II and Umberto I. The latter lies alongside his wife, Queen Margherita.

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"Agrippa finished the construction of the building called the Pantheon. It has this name, perhaps because it received among the images which decorated it the statues of many gods, including Mars and Venus; but my own opinion of the name is that, because of its vaulted roof, it resembles the heavens."

- Cassius Dio (Roman historian writing in the 3rd century AD)