What happened
The Pantheon, a temple to all the gods, was rebuilt in its present form in about AD 113–125, with the work completed under the emperor Hadrian. Its rotunda carries an unreinforced concrete dome about 43 meters across, lit by a circular opening, the oculus, at its crown.
Background
The first Pantheon was built by Marcus Agrippa, the ally of Augustus, in 27–25 BC and was later destroyed by fire. Hadrian’s rebuilding kept Agrippa’s original dedication on the porch, which is why the facade still names Agrippa rather than Hadrian.
Consequences
The dome remains the largest unreinforced concrete dome ever built. Consecrated as a church in 609, the building escaped the ruin of most Roman temples and survives nearly intact, and its form has shaped domed architecture from the Renaissance to modern capitols.