Pantheon dome in Rome with the oculus, a coffered concrete dome, with a central opening to the sky.

The Pantheon (Latin: Pantheon, from Greek Πάνθεον meaning "every god"), an ancient building in Rome, Italy. Built in 118–128 AD. on the site of an earlier building commissioned by Marcus Agrippa during the reign of Augustus (27 BC. - 14 AD.).
The present building was completed by the emperor Hadrian and probably dedicated about 126 AD. He retained Agrippa's original inscription.
It is one of the best-preserved of all Ancient Roman buildings. It has been in continuous use throughout its history, and since the 7th century, the Pantheon has been used as a church dedicated to "St. Mary and the Martyrs" (Latin: Santa Maria ad Martyres) but informally known as "Santa Maria Rotonda".
Almost two thousand years after it was built, the Pantheon's dome is still the world's largest unreinforced concrete dome. The height to the oculus and the diameter of the interior circle are the same, 43.3 metres. The concrete for the coffered dome was poured in moulds, probably mounted on temporary scaffolding. The oculus is the main source of natural light. The height to the oculus and the diameter of the interior circle are the same, 43.3 metres, so the whole interior would fit exactly within a cube (also, the interior could house a sphere 43.3 metres in diameter). These dimensions make more sense when expressed in ancient Roman units of measurement: The dome spans 150 Roman feet; the oculus is 30 Roman feet in diameter; the doorway is 40 Roman feet high. The Pantheon still holds the record for the world's largest unreinforced concrete dome. It is also substantially larger than earlier domes.
The 4,535 metric tons weight of the Roman concrete dome is concentrated on a ring of voussoirs 9.1 metres in diameter that form the oculus, while the downward thrust of the dome is carried by eight barrel vaults in the 6.4 metres thick drum wall into eight piers. The thickness of the dome varies from 6.4 metres at the base of the dome to 1.2 metres around the oculus. The materials used in the concrete of the dome also varies. At its thickest point, the aggregate is travertine, then terracotta tiles, then at the very top, tufa and pumice, both porous light stones. At the very top, where the dome would be at its weakest and vulnerable to collapse, the oculus actually lightens the load.

more on https://en.wikipedia.org/wiki/Pantheon,_Rome