The original Pantheon was built in 27 BC-25 BC under the Roman Empire, during the third consulship of Marcus Vipsanius Agrippa. It was destroyed by fire in AD 80, and the Pantheon was completely rebuilt in about 125, during the reign of the Emperor Hadrian, as date-stamps on the bricks reveal. It was totally reconstructed, with the text of the original inscription (referring to Agrippa) added to the new facade, a common practice in Hadrian's rebuilding projects all over Rome. The building was later repaired by Septimius Severus and Caracalla.
The building is circular with a portico of three ranks of huge granite Corinthian columns (eight in the first rank and two groups of four behind) under a pediment opening into the rotunda, under a coffered, concrete dome, with a central opening (oculus), the Great Eye, open to the sky. A rectangular structure links the portico with the rotunda.
The height to the oculus and the diameter of the interior circle are the same (43 metres, or 142 feet 6 inches), so the whole interior would fit exactly within a cube (alternatively, the interior could house a sphere 43 metres in diameter). The dome is the largest surviving from antiquity and was the largest dome in western Europe until Brunelleschi's dome of the Duomo of Florence was completed in 1436. It was covered with gilded bronze plates.
The interior of the roof is intended to symbolize the heavens. The Great Eye, 27 feet across, at the dome's apex is the source of all light and is symbolic of the sun. Its original circular bronze cornice remains in position. The interior features sunk panels (coffers), which originally contained bronze star ornaments. This coffering was not only decorative, but also reduced the weight of the roof, as did the elimination of the apex by means of the Great Eye.