A striking 2000-year-old temple, now a church, the Pantheon is the best preserved of Rome’s ancient monuments and one of the most influential buildings in the Western world. Built by Hadrian over Marcus Agrippa’s earlier 27 BC temple, it has stood since around AD 125, and while its greying, pockmarked exterior might look its age, it's still a unique and exhilarating experience to pass through its vast bronze doors and gaze up at the largest unreinforced concrete dome ever built.
For centuries the inscription under the pediment – 'M:AGRIPPA.L.F.COS.TERTIUM.FECIT', or 'Marcus Agrippa, son of Lucius, in his third consulate built this' – led scholars to think that the current building was Agrippa's original temple. However, 19th-century excavations revealed traces of an earlier temple and historians realised that Hadrian had simply kept Agrippa's original inscription.
Hadrian's temple was dedicated to the classical gods – hence the name Pantheon, a derivation of the Greek words pan (all) and theos (god) – but in AD 608 it was consecrated as a Christian church and it's now officially known as the Basilica di Santa Maria ad Martyres.
Thanks to this consecration, it was spared the worst of the medieval plundering that reduced many of Rome's ancient buildings to near dereliction. But it didn't escape entirely unscathed – its gilded-bronze roof tiles were removed and bronze from the portico was used by Bernini for his baldachin at St Peter's Basilica. Its exterior is a massively imposing sight with 16 Corinthian columns, each 11.8m high and each made from a single block of Egyptian granite, supporting a triangular pediment. Rivets and holes in the brickwork indicate where the original marble-veneer panels were removed.
During the Renaissance, the building was much studied – Brunelleschi used it as inspiration for his cupola in Florence – and it became an important burial chamber. In the cavernous marble-clad interior you'll find the tomb of the artist Raphael, alongside those of kings Vittorio Emanuele II and Umberto I.
The real fascination of the Pantheon, however, lies in its massive dimensions and awe-inspiring dome. Considered the ancient Romans' greatest architectural achievement, it was the largest cupola in the world until the 15th century and is still the largest unreinforced concrete dome in existence. Its harmonious appearance is due to a precisely calibrated symmetry – its diameter is exactly equal to the Pantheon's interior height of 43.4m. At its centre, the 8.7m-diameter oculus, which symbolically connected the temple with the gods, plays a vital structural role by absorbing and redistributing the dome's huge tensile forces. Rainwater enters but drains away through 22 almost-invisible holes in the sloping marble floor.
Note that tourist visits are not allowed during mass (from 5pm Saturday, from 10.30am Sunday).
An audioguide costs €6.50. There is talk of an admission charge being levied in the future.