World's Largest Unreinforced Concrete Dome by Diameter

The Pantheon's circular hall is supported by a cylindrical masonry wall more than six metres thick, carrying an ancient Roman concrete dome. Unlike modern reinforced concrete structures, this dome achieves its span without embedded steel reinforcement.

The national museums administration in Rome dates the present building to Hadrian's reconstruction of AD 118–125. It is not the same construction phase as Agrippa's first Pantheon, established in 27–25 BC. As an exact completion year is unverified, no precise achievement date is invented.

The inner diameter and interior height are each roughly 43.30 metres, with a circular oculus of about nine metres at the crown. Daylight enters through this opening and rainwater is drained from the floor. These features explain the design but do not define the record metric.