Pantheon, Rome

• 1. Introduction

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  The Pantheon in Rome has been standing for nearly two thousand years, and its concrete dome is still the largest of its kind anywhere on earth. That single fact invites a question most visitors never think to ask: how did a building raised without reinforced steel, without modern engineering software, and without even a clear record of its architect end up outlasting virtually every other structure from antiquity? The dome spans 43.3 metres across, and the height from the floor to the oculus at its crown is exactly the same 43.3 metres. A perfect sphere would fit inside the building with room to spare. On Pentecost, firefighters from the Vigili del Fuoco climb to the roof and drop red rose petals through that open eye in the sky, watching them drift down to the people standing on the ancient marble below. Over six million visitors passed through the bronze doors in 2013 alone. Yet the building was not designed as a tourist attraction. It began as a Roman temple, became a Christian church in AD 609, and has remained in continuous use ever since. That unbroken occupation is the chief reason it still exists. The questions worth pursuing are not just architectural. They concern identity: whose building is this, really? Who built it, who transformed it, who nearly wrecked it, and who saved it?

• 2. Agrippa and Origins

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  Marcus Vipsanius Agrippa began assembling his building complex on the Campus Martius in the years 29-19 BC, in the aftermath of the Battle of Actium in 31 BC. The complex he laid out ran from south to north across his own private land: the Baths of Agrippa, the Basilica of Neptune, and the Pantheon itself. Scholars believe the Pantheon and the Basilica of Neptune functioned as Agrippa's sacra privata, private sacred places rather than public temples. The distinction matters because it may explain why the original building lost its name and purpose so quickly. Adam Ziolkowski has argued that it was in fact originally the Temple of Mars in Campo, a theory that fits the private rather than public religious character of the site.

  Pliny the Elder, writing as an eyewitness, recorded what Agrippa's Pantheon contained. The pillars placed by Agrippa were capped with Syracusan bronze. Diogenes of Athens decorated the building, and the Caryatides he made as columns were regarded as masterpieces. One of Cleopatra's pearls was cut in half, and each piece hung as an earring on a statue of Venus inside the temple. These details give the original building a lustre and specificity that the bare brickwork of the present structure conceals.

  Agrippa's original was destroyed along with other buildings in a fire in 80 AD. Domitian rebuilt it, and then fire struck again in 110 AD. Lise Hetland has argued, by re-examining Herbert Bloch's 1959 brick-stamp study, that the present construction began under Trajan in 114, while the more common view attributes it to Hadrian. Hadrian chose to leave Agrippa's original dedication inscription on the new façade, a practice he followed on other rebuilding projects across Rome. The only building he inscribed with his own name was the Temple to the Deified Trajan. Cassius Dio, writing roughly 75 years after the reconstruction, mistakenly attributed the domed building to Agrippa rather than Hadrian, noting that the building had either received statues of many gods, or that its vaulted roof resembled the heavens.

• 3. Engineering the Dome

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  The concrete dome weighs 4,535 tonnes, and the builders distributed that mass with a precision that still surprises structural engineers. The weight concentrates on a ring of voussoirs 9.1 metres in diameter at the oculus, while the downward thrust travels through eight barrel vaults in the drum wall into eight piers. The drum wall itself is 6.4 metres thick. At the base of the dome the concrete is also 6.4 metres thick; at the oculus it narrows to 1.2 metres.

  The aggregate inside the concrete changes as the dome rises. At the thickest point near the base the builders packed in dense travertine. Higher up they switched to terracotta tiles, then to tufa and pumice, both porous and light, near the summit. Mark and Hutchison's finite element analysis estimated that if ordinary weight concrete had been used throughout, the stresses in the arch would have been roughly 80% greater. The open oculus at the crown is not merely decorative; it eliminates the heaviest point of a dome and lightens the load precisely where the structure would be most vulnerable.

  Hidden relieving arches are built into the mass of the brickwork at the top of the rotunda wall, visible from the outside. Similar arches run over the recesses inside, all of them originally concealed behind marble facing on the interior. The interior floor is not flat; it slopes about 30 centimetres to drain the rainwater that falls through the uncovered oculus during storms. A drainage system below the floor handles the overflow. The Pantheon holds the record for the world's largest unreinforced concrete dome. It is also the only masonry dome that has never required tie-rods, chains, or banding to prevent collapse; all other surviving ancient domes have been retrofitted with such devices or were designed with them from the start.

  The granite columns of the portico came from two quarries in Egypt. The grey columns were cut at Mons Claudianus in the eastern mountains; the pink columns came from Aswan along the Nile. Each shaft is 11.9 metres tall, 1.5 metres in diameter, and weighs 60 tonnes. Workers dragged them more than 100 kilometres from the mountain quarry to the river on wooden sledges, floated them down the Nile during the spring floods, transferred them to Mediterranean vessels, sailed them to the Roman port of Ostia, loaded them onto river barges, and pulled the barges up the Tiber. Even after unloading near the Mausoleum of Augustus, the construction site remained about 700 metres away.

• 4. Conversion and Church Life

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  In 609, the Byzantine emperor Phocas gave the building to Pope Boniface IV. Boniface consecrated it to St. Mary and the Martyrs on the 13th of May 609, and the ceremony was recorded in terms that made plain the symbolic stakes: the pope asked that, after the pagan filth was removed, a church should be made to the holy virgin Mary and all the martyrs, so that the commemoration of the saints would take place where not gods but demons were formerly worshipped. Twenty-eight cartloads of holy relics of martyrs were said to have been removed from the catacombs and placed in a porphyry basin beneath the high altar. Boniface also installed an icon of the Mother of God as Panagia Hodegetria within the new sanctuary.

  This consecration almost certainly saved the building from the fate of most ancient Roman structures. While other monuments crumbled or were stripped for building materials across the Early Middle Ages, the Pantheon remained in use. The saving was not complete, however. When the Emperor Constans II visited Rome in July 663, he spent twelve days pulling down everything of metal that had decorated the city. He stripped the bronze tiles from the church roof and shipped them to Constantinople. The roof remained exposed for nearly a century until Pope Gregory III lined it with lead sheets sometime between 731 and 741.

  By the medieval period the building had accumulated layers of enclosure. A belfry was added on the gable in 1270. Vendor stalls clogged the portico. The ground level of the piazza rose so high from repeated Tiber floods that visitors had to descend down stairs to reach the pronaos until the 17th century. Three granite columns on the eastern end were severely damaged and patched with a brick and rubble wall. It was the popes of the Quattrocento, returning from Avignon, who began serious restoration. Pope Martin V started work on the lead roof; Pope Nicholas V completed it, stamping his papal coat of arms on the roof. Pope Eugene IV cleared the vendors and paved the piazza.

• 5. Renaissance Controversies

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  Raphael was buried in the Pantheon, and the inscription on his sarcophagus was composed by Pietro Bembo: the great mother of all things, Nature, feared to be overcome while he was living, and while he was dying, herself to die. The sarcophagus was given by Pope Gregory XVI. Buried to the right of Raphael lies his fiancée, Maria Bibbiena, who died before they could marry. The painter Annibale Carracci, the composer Arcangelo Corelli, and the architect Baldassare Peruzzi are also interred in the building. Filippo Brunelleschi studied the dome before constructing his own 42-metre dome of Santa Maria del Fiore in Florence, completed in 1436.

  Pope Urban VIII, who reigned from 1623 to 1644, ordered the removal and melting of the massive bronze trusses above the portico. Most of the bronze went into bombards for the fortification of Castel Sant'Angelo. An anonymous Roman satirist posted a pasquinade that has survived as one of the period's most quoted civic gibes: quod non fecerunt barbari fecerunt Barberini, meaning what the barbarians did not do the Barberinis did. Urban VIII's family name was Barberini. The archaeologist Carlo Fea later established from the Pope's own accounts that about 90% of the bronze was used for the cannon.

  Urban VIII also replaced the medieval belfry with twin towers on the intermediate block. These towers were mockingly nicknamed the ass's ears and were not removed until the late 19th century. They were often wrongly attributed to Bernini.

  In 1753, a different kind of damage was carried out in a spirit of improvement. The architect Paolo Posi was hired to restore the attic zone below the dome to conform with Neoclassical taste. Marble and porphyry were stripped out; the windows were made taller and wider and heavily pedimented. Some of the original 64 marble capitals removed from the attic were dispersed into private collections and eventually reached the British Museum. The remodel was condemned across Europe. In the 1930s, a portion of the original attic design was restored.

• 6. Modern Stewardship

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  King Vittorio Emanuele II and King Umberto I, two monarchs of the Kingdom of Italy, are buried in the Pantheon, along with Umberto's Queen Consort, Margherita. The building had been intended as the permanent resting place of the House of Savoy, but when the monarchy was abolished in June 1946, the Italian authorities refused to grant burial to kings who subsequently died in exile. The golden lamp above the tomb of Victor Emmanuel II burns in honor of Victor Emmanuel III, who died in exile in 1947. The National Institute for the Honour Guard of the Royal Tombs, originally chartered by the House of Savoy and now operating under the Italian Republic, still mounts a guard of honour in front of the royal tombs.

  Starting in 1928, a programme of modern restoration began with the aim of minimal structural intervention. The 18th-century coating on the dome was stripped off and replastered. Exterior brickwork was repaired with matching bricks deliberately chiseled with marks to distinguish new work from the original. The floor was polished and failing sections of marble replaced with pavonazzetto and Giallo di Siena. In 1998 the bronze doors, the oldest in Rome, were restored to full functionality after their supporting hinges were replaced. Restoration work in 2005 renewed the patina on the ceiling.

  The building is managed by Italy's Ministry of Cultural Heritage and Activities and Tourism through the Polo Museale del Lazio. Masses are celebrated on Sundays and holy days of obligation, and weddings are held there from time to time. Thomas Jefferson drew on Andrea Palladio's paintings of the Pantheon when designing buildings in the United States. The Pantheon's form directly inspired the Rotunda at the University of Virginia, built so that students of architecture could learn from it. Albert Speer designed the Volkshalle, a planned assembly hall for Adolf Hitler's intended rebuilding of Berlin as Germania, explicitly drawing on the Pantheon's dome, though it was never built.