The Pantheon

In front of you stands the best-preserved Roman building in the world, and it is still in use. The inscription on the pediment reads M·AGRIPPA·L·F·COS·TERTIVM·FECIT: «Marcus Agrippa, son of Lucius, consul for the third time, made this». It's a small deception. Agrippa's temple, from around 27–25 BC, was lost; the one you see was rebuilt by the emperor Hadrian around 118–125, and he kept the founder's name.

1The impossible dome

Inside, a dome 43.3 metres across, exactly the same as its height from the floor, so that a perfect sphere would fit inside, is still the largest in the world made of unreinforced concrete. The Romans lightened it with coffers and with ever lighter materials towards the top. In the centre, the oculus, about 9 metres wide, is the only source of light; when it rains, the water falls inside, and the floor has a gentle slope and small drain holes.

2From temple to church

In 609 Pope Boniface IV consecrated it as the church of St Mary and the Martyrs, with the permission of the Byzantine emperor Phocas. That saved it from ruin. Even so, in 1625 Urban VIII Barberini had the bronze removed from the portico roof. Buried inside are the painter Raphael, who died in 1520, and the Italian kings Victor Emmanuel II and Umberto I.

3What to look for

• The portico columns, each shaft a single piece of granite brought from Egypt.
• The huge bronze doors, ancient though heavily restored.
• The fountain with its obelisk in Piazza della Rotonda, in front of the building.