The Pantheon was built to honor all gods of Rome. It was rebuilt (having previously burnt) by Emperor Hadrian AD 126. The dome measures 142 feet high by 142 feet wide, and was the largest freestanding dome until the 20thcentury.
Completed by the emperor Hadrian c. AD 125, the Pantheon has now stood for almost 1,890 years, one of the most magnificent architectural monuments of antiquity. Even today its domed interior space (the Rotunda) inspires a special awe, not just because of its size (the dome held the world record for a concrete span until the CNIT building in Paris in 1958) but also the quality of the light, the colour, and sound. The building owes its survival partly to the fact that it was converted into a church in AD 608 (St Mary of the Martyrs) but even more to the extraordinary strength and stability of its construction. It was the third ‘Pantheon’ on the site. The first, built by Marcus Agrippa in 27–25 BC, was destroyed in the great fire of AD 80. Replaced by Domitian, it then was struck by lightning in AD 110 and burned again. Plans for rebuilding were probably put in hand immediately by Trajan and work may have been fairly well advanced by the time he died in AD 117 but not actually finished. Hadrian (as was his practice in all the buildings he restored or rebuilt in the city, with the exception of the Temple of Deified Trajan) did not dedicate the new Pantheon in his own name but in that of the original dedicant: thus the bold inscription on the front: M. AGRIPPA L. F. COS TERTIUM FECIT (Marcus Agrippa, son of Lucius, thrice consul, made this). Faintly legible beneath is a two-line inscription in small letters which refers to renovations by Septimius Severus and Caracalla in AD 202: pantheum vetustate corruptum cum omni cultu restituerunt (with every refinement they restored the Pantheum, worn by age) but it seems that no rebuilding was involved. The structure comprises two distinct parts: the front porch and the circular drum, which share the same low plinth (1.3 m, 4½ RF high) but are architecturally in strong contrast—even conflict—with each other. The porch belongs firmly in the Classical tradition of monumental entrances, its pedimented front supported on Corinthian columns with monolithic shafts of Egyptian granite and bases and capitals of white Greek (Pentelic) marble, its exterior also once clad in white marble. The design of the Rotunda, on the other hand, although once coated in white stucco to look like a marble building on the outside, comes from the purely Roman world of concrete bath buildings and palatial halls. (Claridge, Amanda. Rome: Oxford Archaeological Guides, 226-227).