The Pantheon is one of the most famous monuments of ancient Rome on account of its exceptional state of conservation and the beauty and imposing majesty of its architecture. Its layout, circular and preceded by a portico, is a rare departure from the traditional rectangular layout for a classical temple. The name Pantheon (or Pantheum) indicates that the temple was dedicated to many gods, probably the seven planetary divinities.

It was erected by Agrippa (27 BC), rebuilt in its present form by Hadrian and restored by Septimius Severus and Caracalla. In 609 Pope Boniface IV transformed it into the Christian church Sancta Maria ad Martyres and moved here from the catacombs many of the Christian remains of men and women, old people and children who gave their lives as a witness of their faith.

The building has suffered several despoliations in history; the last, under Pope Urban VIII (a Barberini), who ordered the bronze cover over the roof to be removed in order to construct the baldacchino at St Peter's and some guns for the Castel Sant' Angelo. This action prompted Pasquino's famous saying: quod non fecerunt Barbari fecerunt Barberini ('What the Barbarians didn't do, the Barberini did'). Clement IX surrounded the portico with iron railings in 1668. In the days of Pius IX the pavement was partially renovated.

The pronaos or portico is 33.1 metres wide and 15 metres deep. It has sixteen monolithic columns in pink and grey granite, 12.5 metres high and 4.5 metres round, eight along the front and the other eight supporting the roof (three columns on the left were replaced in the 17th century). On the frontal, originally decorated with a bronze relief, is a dedication to the emperor Agrippa: Marcus Agrippa, son of Lucius, Consul for the third time, made this.

You cannot appreciate the Pantheon's importance until you go through its enormous bronze doors into the interior. The proportions are striking; the diameter is equal to the height, 43.3 metres, so that a ball of that diameter could perfectly fit within the Pantheon. It is worth sitting down for a while to enjoy the ancient soul of this unmatched monument: the harmonious proportion of its parts and the solemn austerity of the architectural forms, as well as the even and discreet distribution of light that comes from the single opening: the 8.7 metre-wide skylight. (This skylight is unglazed, and when it rains the central area often gets wet. The floor is slightly concave with a drain at the centre.)

On the walls there are seven great recesses, alternatively rectangular and circular, serving as chapels, with superb monolithic columns and rounded or triangular pediments. The recess opposite the entrance rises above the high altar. In the first chapel on the right there is a painting of the Annunciation attributed to Melozzo da Forli; the second one contains the tomb of Vittorio Emanuele II, first king of Italy, who died in 1878.

On the opposite side are the tombs of Umberto I, murdered in 1900 and queen Margherita, who died in 1926. On the chapel to the right of this (i.e. to the left of the sanctuary) is the tomb of Raphael; it is an old Greek marble sarcophagus with an epitaph full of feeling that says 'Here lies that Raphael by whom, when he lived, Nature 'the great mother of all things' feared to be conquered and to die with his passing'. On the right of the altar is the tomb of Maria Bibbiena, the artist's fiancée, who died before they could marry.