The Pantheon is the ancient Roman temple in Rome famous for its massive unreinforced concrete dome and central oculus. The oculus is an open circular shaft that lets daylight—and rain—into the rotunda below.
The present building was probably completed during Emperor Hadrian’s reign around AD 126, although its inscription credits Marcus Agrippa, who commissioned an earlier temple on the site. The Pantheon’s dome spans roughly 43.3 metres, creating an interior whose height and diameter are nearly equal.
Roman builders reduced the dome’s weight by using heavier aggregate near the base and lighter materials, including pumice, higher up. Coffering further lightens the structure, while the oculus removes the most vulnerable section at the crown. Rainwater drains through openings in the slightly sloped floor.
The building survived unusually well because it was converted into a Christian church in the seventh century. Its portico-and-dome design later influenced architects including Brunelleschi and countless neoclassical designers.