In 609 the ancient Roman temple known as the Pantheon (c. 118-125) was converted into a church, Sancta Maria ad Martyres (also known as Santa Maria Rotonda).
The Pantheon (a temple of all the gods) was originally built by Marcus Vipsanius Agrippa (63-12 BCE), general and close friend and son-in-law of the emperor Augustus. It was completed in either 27 or 25 BCE. A century or so later, in 80, 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, Hadrian had the original inscription, bearing the names of Agrippa and his father, copied onto the new building: M (ARCUS) . AGRIPPA . L (VCIVS) . F (ILIVS) . COS . TERTIUM . FECIT (Marcus Agrippa, son of Lucius, built this in his third consulship).
In time the bronze letters disappeared, but the empty matrices of the inscription made it easy to work out what had once been there. And in 1894 the hollows were filled with new letters.
When the Pantheon became a church a bell tower was added to the pediment. In the 17th century this was replaced by twin bell towers (often mistakenly attributed to Bernini), which were nicknamed the "asses' ears". The bell towers were removed in 1883.
Although the Pantheon is almost 2,000 years old, it still holds the record for having the largest dome of unreinforced concrete in the entire world. The internal diameter of the dome is 44.4m., which is the same as the height from the floor to the oculus.