Above the entrance the inscription 'M·AGRIPPA·L·F·COS·TERTIVM·FECIT' (Marcus Agrippa, son of Lucius, made this in his third consulship) belongs to the first Pantheon of 27 BCE. That building burned. The Pantheon we see was rebuilt from the ground up under Hadrian, around 113–125 CE. Hadrian kept Agrippa's original inscription, preferring to anchor the new building to the empire's founding moment.

The interior rests on two geometries: a hemisphere of 21.7-metre radius, and the full sphere that would fit precisely inside it. The architecture turns a perfect geometric figure into a volume a person can stand inside. The 9-metre oculus at the centre of the dome is the only natural light source; through the day the shaft of light that enters translates the building into a clock.

The engineering revolution lies in the concrete mix itself: the base of the dome is poured with dense basalt aggregate, the crown with lightweight pumice. The structure grows lighter as it rises. The coffered interior is not only decorative but structural, reducing weight at every step. Converted to a Christian church in 609 CE, the Pantheon escaped the dismantling that destroyed most other antique buildings, surviving more nearly intact than any other major Roman monument.