The hole at the top of the dome, the oculus, provides the only light. We owe this marvel of Roman engineering to the emperor Hadrian, who designed it (AD 118-125) to replace an earlier temple built by Marcus Agrippa, son-in-law of Augustus. The shrines that now line the wall of the Pantheon range from the Tomb of Raphael to those of the kings of modern Italy.

The square in which the Roman Pantheon is set (Piazza della Rotonda) was created under Clement XI (1700-21) and involved the demolition of several buildings. At the same time Giacomo della Porta's fountain (1578) was drastically modified: it was given a pedestal decorated whit dolphins and the Pope's coat of arms, and an obelisk was added which, like the one in Piazza della Minerva, came from the neighboring Temple of Isis.

Agrippa's Temple. The plan of the Pantheon combines the pronaos (porch) of a temple whit a rotunda of the kind found in Roman baths. A brilliant composite of geometrical forms and contrasting features, its architecture was intended to reflect the terrestrial and cosmic order.

This is the best preserved building of ancient Rome - thanks to the Byzantine Emperor Phocas' donation of it to pope Boniface IV and its transformation into a church, which received the name of Santa Maria dei Martiri (St Mary of the Martyrs) in 609 AD.

It was originally built in 27 to 25 BC by Agrippa, who wanted to dedicate it to Augustus, his father-in-law and friend. When Augustus declined the honor, it was dedicated to the major deities venerated by the families if Claudius and Julius Caesar (Mars, Venus and the divine Julius himself ) instead.