The Pantheon, a temple dedicated to all the gods, was founded between 25 and 27 BC by Marcus Vipsanius Agrippa, son-in-law of Divus Augustus. Damaged between 80 and 110 AD, it was restored by Hadrian between 118 and 125 AD. Donated by the Byzantine Emperor Phocas to Pope Boniface IV, it was consecrated in 609 as the Basilica of St. Mary ad Martyres.
The pronaos is supported by sixteen Corinthian columns of grey and pink granite. The three columns on the left flank were replaced in the 17th century at the behest of Popes Urban VIII and Alexander VII, using excavated material. The inscription on the pediment reads: M. AGRIPPA. L. F. COS. TERTIUM. FECIT (Marcus Agrippa, son of Lucius, consul for the third time). The gate, 7.53 m high and about 4.90 m wide, has two bronze wings, each weighing 7 tons.
The interior consists of a circular hall with a diameter of 43.30 m and the same height. The wall is interrupted by seven alternating rectangular and semicircular chapels, interspersed with eight aedicules. The attic has a motif of windows and panels made by Paul Posi in 1757. The coffered dome ends in a circular opening of about 9 meters in diameter. The beam of light entering through the dome projects at noon on the axis of the doorway, making the Pantheon an astronomical clock. Rainwater entering through the dome's oculus is drained through 22 holes in the center of the floor.