The Pantheon, one of Rome’s most iconic structures, has stood tall over what is now known as the city's centro storico for nearly 2000 years. Below, a few things you might not know about the building, which draws tens of thousands of visitors each year.

1. THE ORIGINAL PANTHEON BURNED DOWN. 

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Marcus Vispanius Agrippa, a Roman statesman and architect, constructed the Pantheon starting around 27 BCE on his property in the Campus Martius. It was destroyed in a fire, however, around 80 CE, and a second temple built on the site was also ravaged by fire. 

2. IT WAS REBUILT BETWEEN 118-125 CE. 

Nearly 40 years after its initial destruction, the Emperor Hadrian had the structure rebuilt as (presumably) a temple dedicated to the gods. Agrippa’s mark remains, however: an abbreviated inscription on the front of the building reads Marcus Agrippa Lucii filius consul tertium fecit, which translates to “Marcus Agrippa, son of Lucius, Consul for the third time, built this.”

3. IT'S STILL STANDING THANKS TO ROMAN CONCRETE.  

Along with other Roman structures that have withstood the onslaught of nature and time, the Pantheon has remained upright because of the use of a special kind of concrete. A combination of limestone and volcanic ash inside the concrete mix helped form crystals that prevented the spread of microscopic cracks.

4. IT LIKELY SERVED AS A TEMPLE.

The origins of the word Pantheon are Greek and refer to a temple dedicated to all the gods (Pan=all and Theon=gods). The ancient writer Cassius Dio references statues of various gods throughout the interior of the temple, but he himself was unconvinced about this etymology, preferring to think that it was because the vaulted roof “resembles the heavens.” Some modern scholars agree with Dio that the structure may not have been a religious building at all, citing its dissimilarities to other confirmed ancient temples. 

5. IT FEATURES THE LARGEST UNREINFORCED CONCRETE DOME IN THE WORLD.  

Measuring 142 feet in both height and diameter, the large interior dome is one of the highlights of the Pantheon. The thickness of the dome lessens as it climbs, going from 21 feet at the base to 4 feet at the top, thus lightening the stress of the weight of the roof. (In addition, different kinds of concrete were used, ranging from a density of 1600 kilograms per cubic meter to 1350 kilograms per cubic meter.) The dome was the largest constructed in the ancient world. 

6. THERE'S A BIG HOLE IN THE CEILING.

Another remarkable feature of the Pantheon is the oculus, a 27-foot wide opening in the ceiling of the dome. The oculus, which is Latin for “eye,” is the only source of natural light in the interior of the Pantheon.

7. THERE MAY BE A MISTAKE ON THE EXTERIOR. 

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The exterior of the Pantheon includes a portico supported by Corinthian columns, a large pediment with the Marcus Agrippa inscription, and a second pediment attached to the rotunda. But the pediments don’t line up; the first, the Greek prosnaos, is about 13.5 feet shorter than the second, which is in line with the rotunda.

The mistake may be intentional, as the higher pediment lines up perfectly with the obelisk in front of the Pantheon, while the lower pediment matches the height of the obelisk at St. Peter’s Basilica. The most popular theory surrounding the height difference, however, is that the two pediments were going to be the same height until something happened to the planned columns, forcing the builders to compromise with the shorter columns that they had on hand.

8. SOME SCHOLARS BELIEVE THE OCULUS SERVED AS A SUNDIAL.

While most scholars and historians have focused on the structural merits of the dome and oculus, Robert Hannah and Giulio Magli argue that the oculus may have been designed to represent a sundial. Visitors would be able to tell time by the passing of the sun overhead. What's more, the beams that shine through the opening also illuminate the doorway into the rotunda on the March and September equinoxes. 

9. IN THE 7TH CENTURY, IT BECAME A CHURCH. 

In 609 CE, the emperor Phocas gifted the Pantheon to Pope Boniface IV, who then consecrated the building as the Church of Santa Maria ad Martyres. The continued use of the Pantheon is a major reason for its pristine condition 2000 years after it was constructed. To this day, the Catholic Church holds masses and weddings there. 

10. IT HOUSES A LOT OF (FAMOUS) DEAD BODIES. 

Starting during the Renaissance, the Pantheon was used as a tomb for artists like Raphael, Annibale Carracci, and composer Arcangelo Corelli. Italian kings Vittorio Emanuele II and Umberto I, along with Umberto’s wife Queen Margherita, are also entombed in the Pantheon.

11. THOMAS JEFFERSON COPIED ITS DESIGN. 

The architectural majesty of the Pantheon has inspired numerous builders through the centuries. Filippo Brunelleschi modeled the dome of the Santa Maria del Fiore Church in Florence, Italy, after the Pantheon, and Louis XV commissioned Jacques-Germain Soufflot to build the Panthéon in Paris. Construction began in 1822 on the Rotunda on the grounds of the University of Virginia, which was designed by Thomas Jefferson and originally housed a library. It was still under construction upon Jefferson’s death in 1826.