The World's Largest Concrete Dome

Map

Interior of the dome of the Pantheon

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The Pantheon, which was built circa 118 to 128 CE, is the most perfectly preserved temple from classical antiquity.

Almost two thousand years after it was built, the Pantheon's concrete dome is still the largest of its kind in the world. Its internal diameter is 43.3 m (142 ft), which is exactly the same as the height from the pavement to the oculus. 

The dome, which sits on a six-metre thick wall, is made up of five rings, each of 28 coffers that diminish in size as it rises. 

The temple was originally built by Marcus Vipsanius Agrippa (63-12 BCE), general, close friend and son-in-law of the emperor Augustus (r. 27 BCE-14 CE). It was completed in either 27 or 25 BCE. A century or so later, in 80 CE, it was damaged by fire and restored by the emperor Domitian (r. 81-96). It was struck by lightning and burned again in 110 before being rebuilt in its present form during the reign of the emperor Hadrian (r. 117-138).

Completed around 125-128, the new temple curiously bore an inscription to its first patron Marcus Agrippa, which, for centuries, led people to overestimate its age. The inscription reads: M (ARCUS) . AGRIPPA . L (VCIVS) . F (ILIVS) . COS . TERTIUM . FECIT (Marcus Agrippa, son of Lucius, built this in his third consulship). 

At some point the original gilded bronze letters disappeared, but the empty matrices of the inscription made it possible to work out what had once been written. And in 1894 the hollows were filled with new letters.

In 202, the building was repaired  by the emperor Septimius Severus (r. 193-203) and his son Caracalla, who added their own lengthier inscription underneath that of Agrippa. The words are hardly visible today, but, at the time they were carved, the letters may have been painted red to improve their legibility. 
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In 609 the Pantheon was converted into a church, that of Sancta Maria ad Martyres, also known as Santa Maria Rotonda. In time, a small bell tower was added to the apex of the pediment. In the 17th century, Pope Urban VIII (r. 1623-44) had this replaced by twin bell towers (often mistakenly attributed to Bernini), which were nicknamed the 'orecchie d'asino' (ears of an ass). The bell towers were removed in 1883. 

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The Pantheon

Detail of the inscription to Emperor Septimius Severus & his son Caracalla

Interior of dome

Aerial view of the Pantheon

Exterior of the dome

Photograph of the Pantheon with its 17th century bell towers

Drawing of the Pantheon (1530) with original bell tower