The Pantheon's dome is still the largest unreinforced concrete dome in the world, and the inscription across its entrance credits a man who did not build the version standing today.
A dome that has never needed steel
The current Pantheon was rebuilt under Hadrian around 126 CE after fire destroyed an earlier structure, and its dome, cast from unreinforced concrete graded lighter toward the top using different aggregate materials, remains the largest of its kind ever built, a structural achievement that Roman engineering never surpassed and that modern construction rarely attempts to replicate using the same ancient method. The central oculus, an open circular opening at the dome's peak, is the building's only light source and lets rain fall directly onto the floor below, which slopes slightly to drain it away.
Crediting the wrong builder, on purpose
The inscription across the entrance reads that Marcus Agrippa built the temple, even though the surviving structure is Hadrian's later reconstruction rather than Agrippa's original building, a deliberate choice by Hadrian to preserve the earlier dedication rather than claim credit for himself, unusual restraint for an emperor otherwise associated with extensive personal building projects across the empire.
As this entry's companion article covers
The building's conversion into a Christian church in 609 CE, credited with protecting it from the destruction and quarrying that removed most other Roman temples, is addressed directly in this entry's companion article rather than repeated here.