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The Pantheon is a building in Rome that was begun in 27 BC by the statesman Marcus Vipsanius Agrippa, probably as a building of the ordinary classical temple type–rectangular with a gabled roof supported by a colonnade on all sides. It was completely rebuilt by the emperor Hadrian sometime between AD 118 and 128, with some alterations made in the early 3rd century by the emperors Lucius Septimius Severus and Caracalla. It is a circular building of concrete faced with brick, with a great concrete dome rising from the walls and with a front porch of Corinthian columns supporting a gabled roof with triangular pediment. Beneath the porch are huge bronze double doors, 24 feet (7 m) high, the earliest-known large examples of this type.
The Pantheon is remarkable for its size, its construction, and its design. The dome was the largest built until modern times, measuring about 142 feet (43 m) in diameter and rising to a height of 71 feet (22 m) above its base. There is no external evidence of brick arch support inside the dome, except in the lowest part, and the exact method of construction has never been determined.