The Pantheon Latin: Pantheon, from Greek Πάνθεον meaning “every god”) is a building in Rome, Italy, 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, which has confused its date of construction.

The building is circular with a portico of large granite Corinthian columns (eight in the first rank and two groups of four behind) under a pediment. A rectangular vestibule links the porch to the rotunda, which is under a coffered concrete dome, with a central opening (oculus) to the sky. 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 (142 ft). It is one of the best-preserved of all Ancient Roman buildings, in large part because 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”. The square in front of the Pantheon is called Piazza della Rotonda.

Pantheon is derived from the Ancient Greek “Pantheon” (Πάνθεον) meaning “of, relating to, or common to all the gods”: (Pan /”Παν” meaning “all” + Theon / “θεον”= meaning “gods”). Cassius Dio, a Roman senator who wrote in Greek, speculated that the name comes either from the statues of so many gods placed around this building, or from the resemblance of the dome to the heavens. His uncertainty strongly suggests that “Pantheon” (or Pantheum) was merely a nickname, not the formal name of the building. In fact, the concept of a pantheon dedicated to all the gods is questionable. The only definite pantheon recorded earlier than Agrippa’s was at Antioch in Syria, though it is only mentioned by a sixth-century source. Ziegler tried to collect evidence of panthea, but his list consists of simple dedications “to all the gods” or “to the Twelve Gods,” which are not necessarily true panthea in the sense of a temple housing a cult that literally worships all the gods.
In the aftermath of the Battle of Actium (31 BC), Marcus Agrippa started an impressive building program: the Pantheon was a part of the complex created by him on his own property in the Campus Martius in 29-19 BC, which included three buildings aligned from south to north: the Baths of Agrippa, the Basilica of Neptune, and the Pantheon. It seems likely that the Pantheon and the Basilica of Neptune were Agrippa’s sacra privata, not aedes publicae (public temples). This less solemn designation would help explain how the building could have so easily lost its original name and purpose (Ziolkowski contends that it was originally the Temple of Mars in Campo) in such a relatively short period of time.
The concrete for the coffered dome was poured in moulds, probably mounted on temporary scaffolding. The oculus is the main source of natural light.

M·AGRIPPA·L·F·COS·TERTIVM·FECIT
or in full, “Marcus] Agrippa L[ucii] f[ilius] co[n]s[ul] tertium fecit,” meaning “Marcus Agrippa, son of Lucius, made [this building] when consul for the third time.” However, archaeological excavations have shown that the Pantheon of Agrippa had been completely destroyed except for the façade.

The Augustan Pantheon was destroyed along with other buildings in a huge fire in the year 80 AD. Domitian rebuilt the Pantheon, which was burnt again in 110 AD.

The degree to which the decorative scheme should be credited to Hadrian’s architects is uncertain. Finished by Hadrian but not claimed as one of his works, it used the text of the original inscription on the new façade (a common practice in Hadrian’s rebuilding projects all over Rome; the only building on which Hadrian put his own name was the Temple to the Deified Trajan). How the building was actually used is not known. The Historia Augusta says that Hadrian dedicated the Pantheon (among other buildings) in the name of the original builder (Hadr. 19.10), but the current inscription could not be a copy of the original; it provides no information as to who Agrippa’s foundation was dedicated to.