The Pantheon, a temple dedicated to all the gods, was founded between the 25 and the 27 BC. by Marco Vipsanio Agrippa, son-in-law of Divus Augustus. Damaged in 80 and 110 AD, it was restored by Hadrian between 118 and 125. Donated by the Byzantine emperor Phocas to Pope Boniface IV, it was consecrated in 609, with the name of Basilica of S. Maria ad Martyres.
The pronaos is supported by sixteen Corinthian columns of gray and pink granite. The three columns on the left side were replaced in the 17th century by the will of popes Urban VIII and Alexander VII, using uncovered material. The inscription on the pediment reads: M. AGRIPPA. L. F. COS. TERTIUM. FECIT (Marco Agrippa, son of Lucio, consul for the third time did). The door, 7.53 m high and about 4.90 m wide, has two bronze doors weighing 7 tons each. Nothing remains of the original bronze decoration of the pediment.
The interior consists of a circular hall of 43.30 m in diameter and as many in height. The wall is marked by seven alternately rectangular and semicircular chapels, interspersed with eight aedicules. The attic has a motif of windows and panes made by Paolo Posi in 1757. The coffered dome ends in a circular opening of about 9 m in diameter. The ray of light that enters through the dome is projected at noon on the axis of the door, making the Pantheon an astronomical clock. The rainwater that enters from the oculus of the dome is drained through 22 holes located in the center of the floor.