Pantheon, Rome

The Pantheon is an ancient Roman building in the centre of Rome. It consists of three parts: a rectangular porch with columns (the portico), a tall block of brick-faced masonry behind it, and a huge round hall (the rotunda) covered by a dome. From the square outside, the portico looks like a traditional Greek or Roman temple; only when visitors step through the bronze doors do they discover the vast, round, light-filled space inside.

The building we see today was put up in the early second century CE, in the reigns of the emperors Trajan and Hadrian. It replaced an earlier Pantheon built by Marcus Agrippa, the friend, general and son-in-law of the first emperor Augustus, in the 20s BCE. That first building burned and was rebuilt more than once before it was replaced entirely.

The Pantheon is famous for three things. First, its dome, about 43.3 m across, which remained the largest dome in the world for more than 1,300 years and is still the largest ever built of unreinforced concrete. Second, its perfect proportions: the height from the floor to the top of the dome equals the diameter of the rotunda, so an imaginary sphere would fit exactly inside. Third, its survival: it is the best-preserved major building of ancient Rome, still roofed and still in use.

Since 609 CE the Pantheon has been a Christian church, officially the Basilica of Santa Maria ad Martyres (St Mary and the Martyrs). It is part of the UNESCO World Heritage Site covering the historic centre of Rome, and it is one of the most visited monuments in Italy.

Walking into the Pantheon is a journey from the familiar to the astonishing. The portico has eight columns across the front and two more rows of four behind, making sixteen in total. Above them is a triangular pediment, which once held bronze sculpture, and the famous inscription naming Agrippa. This was the language every Roman understood as “temple”.

Between the portico and the rotunda stands a tall rectangular block, often called the intermediate block. It helped join two very different shapes: the straight lines of the temple front and the curved drum of the rotunda. Architects have long noticed that the join is not entirely smooth; for example, the faint outline of a second, higher pediment is visible on the block above the portico roof. This has led to theories that the builders changed their plans during construction.

Inside, the rotunda has a circular floor paved with coloured marble and granite in patterns of squares and circles. The wall of the drum is divided into two levels. The lower level has eight large recesses: the entrance, a great apse opposite, and six others framed by columns. The upper level, or attic, originally had a band of marble panels and small windows; much of what we see today was redesigned in 1747, although a section of the original design was reconstructed in the twentieth century.

Above rises the dome. Its inner surface is covered by five rings of 28 sunken panels called coffers, which get smaller toward the top. They make the dome look lighter, reduce its weight, and lead the eye upward to the oculus, the open eye at the summit. Many scholars have read the dome as an image of the heavens, and the Roman historian Cassius Dio, writing about a century after the rebuilding, already suggested that the building’s name might come from its vaulted roof resembling the sky.

Light is the building’s most important decoration. Because the oculus is the only window, a disc of sunlight moves across the coffers and walls as the sun travels across the sky, turning the building into a kind of giant sundial.

Roman concrete (opus caementicium) was made by mixing lime, water and a volcanic ash called pozzolana, found around Rome and the Bay of Naples, with chunks of stone or broken brick known as aggregate. Pozzolana reacts chemically with lime to make a strong mortar that can set even in wet conditions. This material allowed Roman builders to create vaults and domes on a scale never seen before.

The Pantheon’s builders varied the aggregate according to height. At the foundations and lower walls they used heavy travertine; higher up, broken tile and tufa; and near the top of the dome, very light volcanic stone such as pumice. The dome also becomes thinner as it rises, from about 6.4 m at its base to about 1.2 m around the oculus. Lighter material at the top means less weight pushing outward, which is what causes domes to crack or collapse.

The outside of the dome has a series of step-like rings near its base. These add weight exactly where it is needed to resist the outward push. The drum walls, about 6 m thick, are not solid: they contain hidden chambers and brick relieving arches that channel the load down to the eight main piers and the foundations, a ring of concrete about 7 m wide.

The oculus itself, about 9 m across, is edged by a ring of bricks that acts like a compression ring. Leaving the top open removed the heaviest and most difficult part of a dome to build. Engineers still study the Pantheon because it shows how thoughtful use of materials and shape can make up for the absence of steel reinforcement.

The first Pantheon was built by Marcus Vipsanius Agrippa as part of a building programme on the Campus Martius, the low-lying area north of the old city where he also built baths and other public works. Cassius Dio dates its completion to 25 BCE. He reports that Agrippa wanted to place a statue of Augustus inside and name the building after him, but Augustus refused; so Agrippa set up a statue of the deified Julius Caesar inside and statues of Augustus and himself in the porch. This story shows how closely the building was tied to the new imperial family.

Excavations in the late nineteenth and twentieth centuries revealed remains of the earlier building beneath the present portico. They suggest that Agrippa’s temple faced the same direction, north, but its exact form, and whether it was round at all, is uncertain. Agrippa’s building was damaged by fire in 80 CE and restored under Domitian; it burned again after a lightning strike around 110 CE, in Trajan’s reign.

The inscription on the present façade names Agrippa, not the emperor who actually rebuilt it. According to the Historia Augusta, a late Roman collection of imperial biographies, Hadrian restored many buildings, including the Pantheon, and put the names of their original builders on them rather than his own. For centuries this inscription misled scholars into believing that the whole building dated to Agrippa’s time.

A second, smaller inscription below records a restoration in 202 CE by the emperors Septimius Severus and Caracalla. Unlike Agrippa’s bronze letters, which are modern replacements set into the original sockets, this later text reminds us that the building was maintained and repaired throughout the Roman period.

The main evidence for the date of the present building comes from stamps pressed into the bricks while the clay was still wet. Many Roman brickworks stamped their products with names and sometimes the consular year. In a classic study published in 1959, the German-American historian Herbert Bloch analysed the Pantheon’s brick stamps and showed that most belong to the years around 115–125 CE. This established that the present building is Hadrianic in its completion, not Augustan.

More recently, scholars such as Lise Hetland have argued that the stamps point to construction beginning around 114 CE, under Trajan, with Hadrian (emperor from 117) finishing a project he inherited. On this view the architect might have been Apollodorus of Damascus, Trajan’s great engineer, who designed the Forum and Markets of Trajan. The attribution is attractive but unproven; no ancient source names the Pantheon’s architect. Hadrian himself was a keen amateur architect, and some writers have credited the design to him, also without direct evidence.

The granite columns of the portico reveal an empire-wide supply chain. The grey granite shafts came from Mons Claudianus in the eastern desert of Egypt, the pink ones from Aswan. Each shaft is roughly 11.9 m tall and weighs around 60 tonnes. They had to be dragged to the Nile, shipped down the river and across the Mediterranean to Ostia, and then taken up the Tiber to Rome.

Several scholars have noticed that the portico columns seem too short for the building. The ghost pediment on the intermediate block suggests the designers first planned taller columns of 50 Roman feet, but used shafts of 40 feet. The architectural historian Mark Wilson Jones has argued that the larger columns could not be obtained in time, forcing a redesign; Rabun Taylor has suggested the problem lay in the practicalities of raising such huge shafts on the site. Other explanations, involving changes of plan or the design of the forecourt, have also been proposed. The debate is a reminder that even celebrated buildings were the result of compromises.

Despite its fame, it is not certain what the Pantheon was used for in antiquity. The name, Greek for “of all the gods”, suggests a temple to many or all gods. Cassius Dio himself was unsure: he wrote that it might have been called this because it held statues of many gods, including Mars and Venus, or because its dome resembled the heavens. There is no record of a cult dedicated to “all the gods” with its own priests, and the building never appears in the lists of Rome’s regular state temples.

Dio also records that Hadrian held court in the Pantheon, hearing cases and receiving officials. Many historians therefore see the building as more than a temple: a space where the emperor appeared among images of the gods and of his deified predecessors, expressing the idea that Roman rule was part of a divinely ordered cosmos. The building’s location near other monuments of the imperial family, and its Augustan associations, support this reading.

The play of light has encouraged astronomical interpretations. Some researchers, including Robert Hannah and Giulio Magli, have argued that the building was designed so that the sunlight from the oculus would strike the entrance at noon on particular dates, such as 21 April, the traditional birthday of Rome, when an emperor entering the building would be bathed in light. Others consider such alignments coincidental or impossible to prove. What is clear is that the dome was meant to evoke the sky and that light was part of its design.

For Romans the circle and sphere had strong associations with the cosmos and with perfection. The Pantheon’s geometry, a cylinder topped by a hemisphere enclosing an imaginary sphere, gave these ideas architectural form in a way that visitors could experience with their own bodies.

Many pagan temples in Rome were abandoned, quarried or destroyed after Christianity became the official religion of the empire at the end of the fourth century. The Pantheon escaped this fate. In 609 the Byzantine emperor Phocas gave the building to Pope Boniface IV, who consecrated it on 13 May as a church dedicated to St Mary and all the martyrs. Tradition says that many cartloads of martyrs’ bones were brought from the catacombs and placed under the altar. This made the Pantheon one of the first temples in Rome to be converted into a church.

Conversion did not protect it from every loss. In 663 the Byzantine emperor Constans II, visiting Rome, stripped the gilded bronze tiles from the roof and shipped them toward Constantinople. Pope Gregory III later covered the dome with lead in the 730s. In the Middle Ages the building was fortified, surrounded by houses and shops, and given a bell tower.

In the 1620s Pope Urban VIII, of the Barberini family, removed the bronze girders of the portico roof. Most of the metal was melted down to make cannon for the Castel Sant’Angelo; a popular story that it was used for Bernini’s bronze canopy in St Peter’s is only partly true at best. Romans mocked the act with a famous Latin joke: “Quod non fecerunt barbari, fecerunt Barberini” — “What the barbarians did not do, the Barberini did.” Two small bell towers added to the portico in the same period, nicknamed “the ass’s ears” and long blamed on Bernini, were removed in 1883.

From the Renaissance onward the Pantheon also became a place of burial. The painter Raphael was buried there in 1520 at his own request, followed by other artists and musicians. After the unification of Italy, the first king, Victor Emmanuel II, was buried there in 1878, followed by his son Umberto I and Queen Margherita. The building remains a working church where Mass is celebrated, and since 2023 visitors have paid a small admission fee that helps fund its upkeep.

Because it survived intact, the Pantheon became the most direct model of Roman architecture for later builders. Renaissance architects measured and drew it obsessively. Filippo Brunelleschi studied Roman buildings before building the dome of Florence Cathedral (completed 1436), which has a similar span though a very different structure. Andrea Palladio published detailed drawings of the Pantheon in his Four Books of Architecture (1570), spreading its image across Europe.

The combination of a temple portico and a domed rotunda became a standard formula for churches, libraries, museums and parliaments. Examples include Bernini’s church at Ariccia, St Hedwig’s Cathedral in Berlin, Thomas Jefferson’s Rotunda at the University of Virginia, the Rotunda of Mosta in Malta, and countless university and government buildings. The Panthéon in Paris borrowed the name and the idea of a resting place for national heroes.

The Pantheon also shaped ideas about national and imperial power. In the twentieth century, Albert Speer took it as inspiration for the enormous domed hall planned, but never built, for Hitler’s Berlin. This shows how a single building can be reused to express very different values in different ages.

For engineers the Pantheon remains a test case in the durability of materials. Modern research on Roman concrete, including how it resists cracking and weathering, often begins with the question of how this dome has stood for nearly nineteen centuries without steel.

Chronology. Bloch’s brick-stamp study (1959) established a Hadrianic completion, but brick stamps date the production of bricks, not their use, and stockpiled bricks could be used years later. The distribution of stamps from the 110s has fuelled the argument for a Trajanic start around 114 CE; the dedication is often placed between about 125 and 128. Because Hadrian was in Rome only at certain times, historians also try to connect the project to his travels, with no firm result.

Authorship. The case for Apollodorus of Damascus rests on stylistic and technical similarities with Trajanic buildings, especially the Markets of Trajan, and on the Trajanic start date. Cassius Dio tells a story that Hadrian quarrelled with Apollodorus over architecture and later had him banished and killed, but the reliability of this anecdote is doubtful. The Pantheon may best be understood as the product of a team working within the imperial building administration rather than of a single genius.

The portico problem. The mismatch between the portico and the intermediate block has been analysed through detailed surveys. Mark Wilson Jones’s argument that 50-foot shafts were planned but 40-foot shafts used is widely cited, but some scholars question whether the building was truly “flawed” or whether the arrangement was intended. The debate illustrates a methodological point: reconstructing design intentions from the finished building requires careful attention to building sequence, masonry joints and the logistics of materials.

Function and meaning. Interpretations range from a conventional temple to many gods, to a dynastic shrine linked to Augustus and the deified emperors, to an imperial audience hall, to a cosmological monument. These are not mutually exclusive. The volume edited by Tod Marder and Mark Wilson Jones (2015) brings together recent work on these questions, as well as on the building’s later history.

Written sources on the Pantheon are few. Cassius Dio (early third century) provides the main account of Agrippa’s building and of Hadrian’s use of the new one; the Historia Augusta mentions Hadrian’s restoration; later Christian sources such as the Liber Pontificalis record the conversion under Boniface IV and the removal of the bronze tiles by Constans II. Everything else must be deduced from the structure itself.

The building has therefore been studied with the tools of architectural archaeology: measured surveys, analysis of construction joints, brick stamps, the petrology of the granite and marble, and excavation around and beneath the portico. Since the late twentieth century, laser scanning and computer modelling have allowed researchers to test structural hypotheses, such as how the dome distributes its loads and how cracks that formed in antiquity affect its stability.

The dome has developed cracks, probably from early settlement and from earthquakes, yet it remains stable. Engineers explain this partly by the way the dome behaves as a series of arches once cracked, held by the heavy ring of the drum and the step-rings at its base. Modern conservation has included the removal of later coatings in the 1930s, repairs to the roof and consolidation of interior plaster in the early 2000s, and ongoing monitoring of structural movement.

Research on Roman concrete, including studies of lime inclusions and the chemical reactions of volcanic ash, has attracted attention as a possible guide to more durable and less carbon-intensive modern materials. Care is needed, however: the Pantheon’s survival also depends on its constant use and maintenance, on the absence of rusting steel, and on the fortunes of history, not only on the chemistry of its concrete.