Pantheon, Rome: Hadrian's Temple and Its Great Concrete Dome

The Pantheon is a Roman temple in the centre of Rome, rebuilt in its present form between about AD 113 and 125 under the emperors Trajan and Hadrian. Its circular hall is covered by a coffered concrete dome 43.3 metres across, open to the sky at the top, which remains the largest unreinforced concrete dome in the world. Converted into a church in AD 609, it is the best-preserved major building of ancient Rome and the model for countless domes from the Renaissance to the present.

Agrippa’s first Pantheon

The first Pantheon was built by Marcus Vipsanius Agrippa, the general and son-in-law of Augustus, as part of a programme of public buildings on the Campus Martius, the low ground beside the Tiber. It was put up during Agrippa’s third consulship, in 27 BC, and finished by about 25 BC. Little is known of its form. Excavations in the late 19th century suggested it was a rectangular building facing south, but work in the 1990s indicated that it faced north, like the present one; its exact plan is still debated.

Agrippa’s building was damaged in the great fire of AD 80 and restored under Domitian. According to later sources, it burned again in AD 110, possibly after a lightning strike, during the reign of Trajan. This second disaster led to a complete rebuilding.

The rebuilding under Trajan and Hadrian

For a long time the present Pantheon was attributed entirely to Hadrian, who reigned from AD 117 to 138. Study of the stamps impressed on the bricks, which often record the year they were made, has changed this picture. Most date from about AD 114 to 123, suggesting the project began late in Trajan’s reign and was finished under Hadrian by about 125. Some scholars think Trajan’s architect Apollodorus of Damascus produced the design, but there is no direct evidence.

Hadrian did not put his own name on the building. Instead, the frieze of the portico carries the inscription M·AGRIPPA·L·F·COS·TERTIVM·FECIT, “Marcus Agrippa, son of Lucius, consul for the third time, made this”. This respectful gesture to the original founder later misled scholars into dating the whole building to Augustus’s time. A smaller inscription below records a restoration by Septimius Severus and Caracalla in AD 202.

Hadrian seems to have valued the building. Cassius Dio reports that he held court there, which suggests it served as an imperial audience hall as well as a temple. Its precise religious function, and which gods were honoured there, remains uncertain.

Design: portico, rotunda and dome

The Pantheon combines three parts. At the front is a deep porch, or portico, with 16 monolithic columns of Egyptian granite, each shaft about 11.9 metres (39 feet) tall and weighing roughly 60 tonnes. Grey granite from Mons Claudianus in the Eastern Desert of Egypt was used for the front row and pink granite from Aswan for the others. Behind the portico, a rectangular transitional block links it to the rotunda.

The rotunda is a cylinder of brick-faced concrete about 6 metres thick, hollowed by niches and hidden chambers that reduce weight. Above it rises the dome. Its interior diameter and the height from floor to crown are the same, about 43.3 metres, so the space could contain a perfect sphere. At the top is the oculus, an open circle about 8-9 metres across, which is the building’s only source of natural light apart from the door.

The dome’s underside is decorated with five rings of 28 sunken panels, called coffers. They lighten the shell and create a play of light and shadow as the sun’s disc moves around the interior through the day. The great bronze doors at the entrance are ancient, though they have been restored.

The interior is lined with coloured marble brought from across the Roman world: yellow giallo antico from Numidia in North Africa, purple-veined pavonazzetto from Phrygia in Asia Minor, red porphyry and grey granite from Egypt. The floor repeats these stones in a pattern of squares and circles, much of it original or carefully restored.

At ground level, the wall opens into seven large recesses plus the entrance bay, alternately rectangular and semicircular, framed by columns, with smaller shrines called aediculae between them. Statues of gods, and probably of deified emperors, once stood here; today the recesses hold Christian altars and tombs. Above runs an attic zone, which was remodelled in 1747; a short section was restored to its supposed ancient appearance in the 1930s.

Some puzzles remain. The awkward junction between the portico and the rotunda, where two cornices do not quite meet, has led scholars such as Mark Wilson Jones to suggest that the portico columns were originally meant to be taller, and that the builders adapted the design when larger shafts could not be supplied. This is an attractive theory, but it cannot be proved.

How the dome stands

The dome is built of Roman concrete, a mixture of lime mortar, volcanic ash and pieces of stone or brick called aggregate (see Roman concrete). The builders graded the aggregate: heavy travertine and tuff in the lowest levels, then tuff and brick, and light volcanic pumice and scoria near the top. The shell is about 6.4 metres thick at its base and thins to about 1.2 metres around the oculus.

Outside, a series of step-rings around the lower part of the dome adds weight where it is most needed to resist the outward thrust. A brick and stone ring frames the oculus and acts as a compression ring. The dome contains no steel reinforcement, which modern concrete would require, and it has stood for almost 1,900 years. Cracks developed in antiquity, but the structure found a stable balance.

From temple to church

After the end of pagan cult in the 4th century, the Pantheon was closed. In AD 609 the Byzantine emperor Phocas gave it to Pope Boniface IV, who consecrated it as the church of Santa Maria ad Martyres, Saint Mary and the Martyrs. Tradition holds that cartloads of martyrs’ bones were brought from the catacombs and placed under the altar. Its conversion is the main reason it survived when many other temples were quarried for stone.

The building was still stripped of valuable metal. In AD 663 the visiting emperor Constans II removed gilded bronze tiles from the roof. In 1625 Pope Urban VIII, of the Barberini family, had bronze taken from the portico ceiling, reportedly for cannon for Castel Sant’Angelo and for work at St Peter’s. A famous Roman joke followed: “Quod non fecerunt barbari, fecerunt Barberini”, “what the barbarians did not do, the Barberini did”. Urban VIII also commissioned two small bell towers, nicknamed “the ass’s ears”, which were removed in 1883.

The Pantheon became a burial place for distinguished people. The painter Raphael was buried there in 1520 at his own request. After Italian unification it received the tombs of King Victor Emmanuel II (died 1878), King Umberto I (died 1900) and Queen Margherita.

Influence on architecture

Few buildings have been copied as often. Renaissance architects, including Filippo Brunelleschi, studied its dome; Andrea Palladio published measured drawings of it in 1570. Its form of portico plus domed rotunda inspired the Panthéon in Paris, Thomas Jefferson’s Rotunda at the University of Virginia, the Rotunda of the British Museum’s Reading Room and many libraries, churches and state buildings. The dome of Hagia Sophia took Roman domed construction in a different direction, but builders in both East and West looked back to the Pantheon as the benchmark.

Myths and facts

Myth: Agrippa built the Pantheon we see today. The inscription says so, but the present building dates from the 2nd century AD; Agrippa’s temple was destroyed by fire.

Myth: Rain never falls inside because of rising warm air. Rain does fall through the oculus. The floor slopes slightly and small holes in the marble drain the water away.

Myth: The Pantheon’s secret concrete recipe is lost. The ingredients are well understood. What impresses engineers is the careful grading of materials and the proportions of the structure.

By the numbers

Timeline

• 27-25 BC: Marcus Agrippa builds the first Pantheon on the Campus Martius.
• AD 80: Fire damages the building; Domitian restores it.
• AD 110: The Pantheon burns again during Trajan’s reign.
• c. AD 113-125: The present building is constructed under Trajan and Hadrian.
• AD 202: Septimius Severus and Caracalla restore the building.
• AD 609: Pope Boniface IV consecrates it as Santa Maria ad Martyres.
• 1520: Raphael is buried in the Pantheon.
• 1625: Urban VIII removes bronze from the portico.
• 1883: Bernini-era bell towers are demolished.

Why it matters

The Pantheon shows Roman engineering at its boldest and has survived almost unchanged in structure. It links the age of Augustus, the reigns of Trajan and Hadrian, the medieval papacy and modern Italy, and it set the pattern for the domed buildings of the Western world. Hadrian’s interest in daring vaulted spaces can also be seen at his country estate, Hadrian’s Villa at Tivoli.