The Pantheon is one of the most impressive historical and architectural sites in the centre of Rome, Italy. Dated around the early second century AD, it is one of the best preserved building of antiquity in the world, and testifies the superiority of Roman building techniques. Its dome, with its distinctive central hole, the “oculus”, is the biggest ever built in masonry.

The Pantheon is a Roman temple which was presumably dedicated to all the gods, as its Latin name Pantheum, comes from the ancient Greek (Πάνθειον) Pantheion, which means “[the temple] of all the gods”. People from all over the world visit Rome to admire its near perfect state of preservation, its regular mathematical proportions and its huge masonry dome (the biggest ever built), as well as to discover its mysterious and peculiar history. This extraordinary architectural and engineering achievement of the ancient world inspired many later buildings and has influenced the entire history of Western architecture.

The History of the Pantheon

The legend of the first Pantheon

According to legend a temple was first built here in the seventh century BCE in the Campus Martius (or “Field of Mars”) a large area of about 2 square kilometres where religious festivals and military musters were held, located to the north of the original site of Rome and its seven hills.

The site corresponded to a marsh called Palus Caprae (the pool of the goat) where Romulus, the mythical founder of Rome, who was son of the god Mars, supposedly ascended into the sky during a thunderstorm while he was reviewing the army ¹. Although a temple probably did not exist here at such an early date it seems that there was a small altar dedicated to Mars in the vicinity.

Agrippa’s Pantheon

The first historically documented construction of the Pantheon was begun in 27 BCE by Marcus Vipsanius Agrippa (63 BCE – 12 BCE) a military commander and close collaborator of the Emperor Augustus. The architect was probably Lucius Cocceius Auctus and the building was perhaps completed in 25 BCE. Excavations in the nineteenth century and from 1995 to 1997 showed that there is a lower floor, about 1.5 meters below the current floor, which may correspond to this original Augustan building or to its replacement or restoration by Emperor Domitian after it was damaged by fire in 80 CE. The exact form of Agrippa’s Pantheon is still debated.

Some archaeologists think it had a north-facing entrance consisting of a portico looking onto a paved square (like the present building), while others believe that its entrance was to the south (the other side compared to its present location).

This south entrance would have adjoined the Basilica of Neptune and the site of Agrippa’s public baths (laconica), the first of their kind to be built in Rome. The original Pantheon was probably also circular, with an internal colonnade delimited by a wall in opus reticolatum.

This typically Roman masonry technique was often used for the perimeter walls of funerary monuments. It consists of pyramidal stone blocks with the pointed ends embedded in a wall of cement, so that their square bases form a diagonal pattern. Agrippa’s Pantheon seems to have been richly decorated.

In book 34 (chapter 7) of Pliny’s Natural History (written prior to the building’s destruction in 80 C.E.) we read that “the capitals of the pillars in the Pantheon erected by Marcus Agrippa are of Syracusan bronze”. According to Pliny: “the Pantheon of Agrippa was embellished by Diogenes of Athens. Among the supporting members of this temple there are Caryatids that are almost in a class of their own, and the same is true of the figures on the angles of the pediment, which are, however, not so well known because of their lofty position.”

Hadrian’s Pantheon

The fire of 110 and the rebuilding of the Pantheon

It is historically documented that in 110 C.E., only 30 years after Domitian’s restoration of the building, the Pantheon was again burnt down by a fire, this time caused by lightning, and it had to be rebuilt. There are three main theories concerning the rebuilding of the Pantheon:

1. It was merely restored by Hadrian (emperor from 117 to 138 AD), as stated in the Historia Augusta (“Augustan History”) dating to the mid-fourth century: “He built countless public buildings everywhere, but he inscribed his own name on none of them except for the temple dedicated to his father Trajan. In Rome he restored the Pantheon, the voting-enclosure (Septa), the Basilica of Neptune (…), the Baths of Agrippa, and dedicated them in the names of the original builders.” ³
2. It was rebuilt over a long period, starting under Emperor Trajan (reigning from 98 to 117 AD) and ending under his successor Hadrian.
3. It was perhaps restored by the architect Apollodorus of Damascus on the orders of Trajan, but then demolished and completely rebuilt by Hadrian.

The most recent research, backed up by evidence from excavations carried out in the late nineteenth century, tends to support the third theory. The foundations had to be reinforced so perhaps they were not intended to support the building that we can see today.

The restored bronze inscription above the colonnade of the portico: “M·AGRIPPA ·L·F· COS·TERTIVM· FECIT”  indicates Agrippa as the maker of the temple, but this is certainly a tribute by Hadrian to the original builder, as the Augustan History claims. It is unlikely that Agrippa would have neglected to mention which gods the building was dedicated to or that he would have referred to himself as “consul tertium” (consul for the third time), which is his honorary тιтle found on commemorative coins made after his death.

Following the rise of Christianity and the end of pagan worship, the Pantheon seemed doomed to the abandonment, destruction, and spoliation that were the usual fate of ancient Rome’s great temples and public edifices. But the building received a new lease of life when the Byzantine emperor Phocas officially gave it to the Catholic church.

According to tradition it was consecrated on May 13, 609 and it soon became known as Sanctae Mariae ad martyres, being dedicated to the Virgin Mary and to the Christian martyrs. Thus the building was saved and preserved, due to its new function as a church, and it is still used as a place of Christian worship.

Description

The structure of the Pantheon consists of two main architectural elements:

1. The main building (known as the “rotonda”) with a circular ground plan and a thick windowless wall in which there are 7 large niches at ground level (8 if one includes the entrance). This wall is surmounted by a hemispherical dome with a large central hole (the “oculus”).
2. A projecting portico or porch, faced by a façade consisting of 8 columns at the front and two at the sides. This structure gives access to the door to the main building.

The Foundations

The foundations of the Pantheon are made of concrete consisting of layered pieces of travertine stone held together by lime mixed with pozzolana sand. They were originally 4.7 meters deep and 7.3 meters thick but during the construction of the building the foundations cracked, due to the underlying marshy land consisting of unstable river clay. (As mentioned above they were perhaps not intended to support the larger building erected under Hadrian).

For this reason, a second reinforcement foundation ring was built, projecting 3 meters beyond the original perimeter. In addition thick ʙuттress walls were built on the south side of the building, opposite the porch and they were firmly anchored to the adjoining Basilica of Neptune. This had the effect of stabilizing the structure by counterbalancing the forces and weights at either end of it.

The Structure of the “Rotonda”

Height of wall	30.40 metres (external) 21.70 metres (internal)
Thickness of wall	6.40 metres at the base

The “rotunda” (Italian for “round”) is the name for the main central structure of the Pantheon and it refers to the Pantheon’s distinctive cylindrical shape. This was the first element of the building to be constructed, using the Roman concrete (opus caementicium) technique.

Hydraulic cement made of lime mortar mixed with volcanic ash and aggregate was sandwiched between horizontal layers of bricks every 1.2 meters and outer facings of bricks (opus latericium). The external wall is clearly divided into 3 sections, each divided from the next by a projecting cornice, now mostly reduced to a succession of projecting marble blocks known as “modillions”. The composition of the concrete changes depending on the height of the wall, so that there are three different types of concrete:

1.  From the floor up to the first cornice: layers of cement packed with travertine and volcanic tuff fragments
2.  From the first to the second cornice: alternating layers of volcanic tuff and broken tiles or bricks embedded in the same cement.
3.  From the second cornice onwards: cement predominantly containing crushed and broken bricks.

In this way the higher parts of the wall were lighter, as they did not need to bear so much weight. The wall is also like a honeycomb as it contains many cavities on all its levels, which, together with the large internal niches, helped to reduce the quanтιтy of materials used and the overall weight, which is nevertheless immense due to the thickness of the walls which reaches as much as 6.40 metres.

Each internal niche is separated by mᴀssive piers or pillars and above each of these openings the wall is strengthened and supported by a semicircular archway of bricks, known as a relieving arch. These thin bricks stand on end and can be clearly seen on the exterior wall. They effectively distributed the weight of the concrete onto the piers below while the concrete dried and solidified, a process that took many years.

The Dome

The dome of the Pantheon is the most remarkable part of a remarkable building. With an internal diameter of 43.44 meters, almost half the length of a football field, it is the largest dome ever built in masonry or unreinforced concrete. It thus represents an enduring demonstration of the genius of the Roman architects, as the building is still intact today. It was made of concrete mixed with volcanic tuff (tufo) and pumice stone.

Diameter	43.40 meters
Height	21.75 meters
Weight	c. 5,000 metric tonnes
Maximum thickness (at the base)	5.90 meters
Minimum thickness (at the top)	1.40 meters

Externally the bottom part of the hemispherical dome (corresponding to the first two rings of coffering) is encased by a thick upper section of the wall which contains the outward thrust of the dome. Above this there are seven concentric rings that look like steps rising around the side of the dome, which appears quite flat from the outside.

Inside the building the curvature of the dome is more evident, and its diminishing diameter is emphasized by the five rings of 28 coffered indentations or sunken panels that become progressively smaller as they approach the central hole or oculus. The bronze roof tiles that originally covered the dome have been replaced with lead sheeting.

How was the dome built?

In order to build the dome it was necessary to have a stable support for the wooden frames of the centering, or formwork, onto which the concrete was laid. When it had hardened these supports were then removed. Various ideas have been proposed as to how these temporary structures were held in place.

They very probably rested on the cornice at the top of the interior wall, which is a projecting ring that runs all around the rotunda. Some scholars propose that opposing sections of formwork were lifted into place by mᴀssive cranes and that they supported and balanced each other, resting on a central ring corresponding to the oculus above, but it is far more likely that their upper parts were firmly supported by a central tower made of wooden scaffolding.

The siege towers used by the Roman army were very similar structures and this would have been a more practical and versatile solution for the following reasons:

• It would have been much safer and more stable than precariously counter-balanced structures.
• The central tower could have been used to lift materials and workers to any level of the dome as it rose.
• The formwork would have been easier to move, to position precisely and to remove.
• The same pieces of formwork could have been reused as new layers and sections of concrete were laid next to each other.

Due to the immense weight of the dome it was necessary to make it progressively lighter as it rose. This was done by reducing its thickness from around 5.90 meters at the bottom to 1.40 meters at the central oculus. Also the deep internal coffering of the dome certainly helped to make it lighter. In addition the composition of the concrete aggregate was altered by mixing different materials into hydraulic cement made of lime and black volcanic pozzolana sand.

Alternating layers of bricks and tufa at the bottom were thus replaced by large lumps of light tufa, porous volcanic slag and pieces of volcanic pumice in the upper dome above the external step-rings. Finally, at the top, small clay pots were inserted to make holes in the concrete.

This solution would be used in several other Roman domes and vaults, for example two centuries later in the Baths of Diocletian, followed by the Basilica of Maxentius and the Mausoleum of Helena, where the technique is now clearly visible due to extensive damage to the dome. Due to the fragments of the vases or amphorae embedded in the dome this latter building has given its name to the surrounding residential district of Torpignattara (meaning “Tower of cooking pots”).

The interior of the dome

On the internal surface of the dome there are five rows of 28 square sunken panels, technically known as coffers, but also called caissons (‘boxes”), or lacunaria (“spaces”). This is a very unusual way to divide a circle, and it would have been difficult to calculate the correct spacing between the coffers. Twenty-eight days amount to four weeks and this is almost the same as the time it takes for the Moon to return to the same position among the stars, which is defined as a sidereal month (approximately 27 days and 8 hours).

In addition it is also one of the four “perfect” numbers known in antiquity (6, 28, 496, and 8,128) of which the sum of the factors equals the number itself ( 1 + 2 + 4 + 7 + 14 = 28). The Greek mathematician Pythagoras believed that such numbers had a mystical meaning expressing harmony with the cosmos.

Since the oculus at the top of the dome was the only source of external illumination, with a circular sunbeam marking time during the day as it moved across the floor and wall of the rotunda, the building can be seen as expressing a cosmic union and harmony between the sun and the moon. It has also been supposed that seven altars, positioned in the large niches on the ground level of the building may have been dedicated to the gods corresponding to the seven celestial bodies of the solar system that were known in Roman times.

In book 53 chapter 27 of his Roman History Cᴀssius Dio speculated that the Pantheon took its name either from the many statues of the gods that it was adorned with, or because its dome resembles the heavens. Thus the building can be seen as a microcosm of the divinely ordered (and divinely ordained) Roman world, with the emperor presiding over it all, in conformity with the celestial movements.

In Marguerite Yourcenar’s imaginary autobiography of Hadrian Memoirs of Hadrian (Mémoires d’Hadrien) the emperor says the following words as regards the Pantheon: “My intention was that this sanctuary of All Gods should reproduce the terrestrial globe and the stellar sphere, that globe which encloses the seeds of eternal fire, that hollow sphere containing all. (…) The hours were to circle the centre of its carefully polished pavement where the disk of daylight would rest like a shield of gold; rain would form a clear pool on the floor, from which prayers would rise like smoke toward that void where we place the gods.”

The 140 coffers were made by pouring concrete onto large wooden formwork blocks. Their function was not only aesthetic, but they had a fundamental role in making the dome lighter and they also accelerated the evaporation of water and the solidification of the cement mix by increasing the surface area exposed to the air during the process of carbonatation (the chemical reaction between calcium hydroxide in the cement and carbon dioxide in the air to form calcium carbonate). The interior of the coffers may have originally been decorated with ornaments such as bronze stars or rosettes.

The Oculus

At the top of the dome there is a large central opening with a diameter of 8.8 meters called the “oculus” (the Latin word for “eye”), which is the only source of natural light in the building apart from the entry door. A beam of light penetrates this circular hole to create a dramatic effect as it slowly travels across the floor and wall of the rotunda, illuminating them like a spotlight. This intriguing aspect of the Pantheon immediately catches the eye of any visitor who enters the vast circular space.

The oculus also allows cool air to enter the building in the H๏τ summer months, and it even helps to keep the floor clean, since rain or snow falls through it and flows into a drainage system through small holes in the marble floor. The oculus acts as a compression ring that distributes the forces of compression at the centre of the dome. It is 1.40 meters thick at the edge and is reinforced by 3 successive rings of long flat bricks known as “bipedales”. The bronze ring that covers its edge dates back to the original construction, and is all that remains of the original bronze roofing.

The Exterior

At the front of the Pantheon there is a portico or pronaos which is typical of the entrance to a Greek temple. This porch measures 34.20 by 15.60 meters and a total number of 16 monolithic columns are arranged so as to form a central pᴀssage and two lateral aisles, with the 8 columns of the façade supporting a triangular pediment. These frontal columns are made of grey granite from the Mons Claudianus quarry in the eastern desert of Egypt, while the internal columns are made of a pinkish granite from Aswan.

Each column is 11.8 meters high and weighs around 60 tons. Their bases and Corinthian capitals are made of white “Pentelic” marble from the quarries on Mount Pentelicon near Athens (the same marble that was used to build the Parthenon). At some unknown date three grey columns on the left (eastern) side were removed. They were replaced in the 17th century, but the reddish hue of these columns is not a very good match.

Today the tympanum (the space in the centre of the pediment) is bare and empty, but the holes marking the location of clamps suggest that it originally contained a high relief sculpture, probably made of gilded bronze. It has been plausibly suggested that this was an imperial eagle standing within a wreath or crown of oak leaves, perhaps with decorative ribbons extending into the corners of the pediment.

The eagle symbolized the power of Jupiter and the Roman Empire, but also alluded to the apotheosis of mortals into the immortal realm. Although this hypothetical eagle has long since flown the previously mentioned inscription in bronze letters “M[arcus] Agrippa L[ucii] f[ilius] co[n]s

• tertium fecit,” (Marcus Agrippa, son of Lucius, made [this building] when consul for the third time) still survives on the architrave below the pediment.

 

Closer examination reveals another inscription cut into the stone of the architrave, just below this much larger text. It commemorates repairs carried out by the emperor Septimius Severus and his son Caracalla in 202, stating that they “carefully restored the Pantheon which was damaged by age” (PANTHEVM · VETVSTATE · CORRVPTVM · CVM · OMNI · CVLTV · RESтιтVERVNT).

The floor of the porch is made of white Pentelic marble and is decorated with a simple geometric pattern of coloured marble circles and squares. The roof of the porch was originally held up and reinforced by bronze trusses, which were removed by pope Urban VIII of the Barberini family to cast cannon for the protection of Castel Sant’Angelo (another building closely ᴀssociated with Hadrian), and replaced with timber beams. This occasioned the famous Pasquinade Quod non fecerunt Barberi, fecerunt Barberini (“What the barbarians did not do, the Barberini did”).

The porch of the Pantheon and the rotunda are connected by an intermediate or transitional block, which basically consists of two wedges that adapt the rectilinear geometry of the porch to the circular geometry of the rotunda. It contains a pair of staircases that give access to the roof and its external marble walls are decorated by fluted pilasters and a series of three-foot-tall bas relief friezes of candelabra, ribbons and festoons, with various utensils used in religious ceremonies.

If the Pantheon is viewed from a distance it is clear that there is the outline of a second higher pediment where the roof of the porch joins the intermediate block in front of the rotonda. This strange feature may be explained by the theory that the original intention was to use granite columns with shafts 50 Roman feet tall and capitals 10 Roman feet tall, instead of the smaller shafts of 40 Roman feet and capitals of 8 Roman feet that now exist.

They would have corresponded to the second pediment visible on the front of the intermediate block. For some reason (possibly a shipwreck) these columns failed to arrive, and the builders had to use the smaller columns that still exist today.

This idea is supported by the recent discovery of an ancient set of full scale plans and templates for the portico of the Pantheon cut into the limestone paving next to the Mausoleum of Augustus, only 600 meters to the north.

These templates were probably used for checking and shaping building materials that were brought to this site, after they were unloaded from nearby docks on the Tiber. In fact the dimensions of these templates correspond to that of the original larger columns for the portico.

Excavations carried out in the square (Piazza della Rotunda) in front of the Pantheon have revealed that the ancient street level was around two meters below the present level. It is therefore clear that a flight of steps originally led up to the portico of the building. Due to the periodic flooding of the Tiber in this area, as well as deposits of rubbish caused by human habitation and the periodic demolition or collapse of buildings, the ground level around the Pantheon, as well as around most other ancient Roman buildings and monuments, steadily rose over the centuries.

Interior

Upon pᴀssing through the doors at the back of the portico one enters the single inner room or cella of the temple, a circular hall covered by a vast hemispherical dome. This engineering tour de force has exerted an enormous influence on the history of Western architecture as it seems to have been deliberately designed to surprise the visitor, due to its interior that vastly surpᴀsses the exterior in splendour. The two mᴀssive bronze doors measuring 12 x 7.5 meters are not original (in fact they are too small for the door frame) and probably date to the Middle Ages.

The interior of the rotunda is a cylinder, measuring 43.44 meters in diameter (corresponding to 150 Roman feet). There is exactly the same distance from the floor to the middle of the oculus at the top of the dome.

The building is therefore based on the dimensions of a perfect sphere. In addition if we visualize a square fitting exactly into the cylinder of the rotunda, with an identical square standing immediately next to it towards the north, this second square would extend to the corner columns of the portico (with its sides exactly corresponding to the centres of the outer columns) and its height would be the same as that of the transitional block (as well as of the top of the pediment as originally planned).

The wall of the rotunda is 6 meters thick and is punctuated by seven deep niches or alcoves, the ceilings of which are supported by two Corinthian columns. Three of them have a semicircular floor-plan (the one on the main axis directly opposite the doorway and those on either side of the building on the axis at right angles to this) and the other four (on the 2 diagonal axes) have a rectangular floor-plan. Perhaps these niches were dedicated to the seven divinities ᴀssociated with the sun, the moon and the five planets that were known at the time: Mercury, Venus, Mars, Jupiter and Saturn.

The niche opposite the doorway is the most impressive, as it is the only one that extends above the level of the first internal cornice (like the arch above the doorway). It is flanked by two Corinthian columns of yellowish pink giallo antico marble from Tunisia. In the columns of the other alcoves this stone alternates with ivory-coloured and purple-veined pavonazetto from Turkey.

The richly coloured highly-prized marbles used for these columns, as well as on the walls and in the eight aedicules attached to the mᴀssive piers between the niches, came from all over the Mediterranean (modern-day Egypt, Greece, Turkey, and North Africa), thereby reminding visitors of the vast extent of Rome’s dominions, their great variety and their wealth. Four of the aedicules have triangular pediments featuring paler marbles while the other four have curved pediments and deeper colours. Unfortunately the original columns on these aedicules made of precious purple porphyry have been removed and replaced with shafts of rosso antico or gray granite.

The only local Italian stone in the original decorations of the Pantheon is the fine white marble from Carrara in Tuscany, which was used for the Corinthian capitals and the small pediments of the aedicules. It was no doubt chosen due to the fact that it can be carved in exquisite detail. The extraordinarily precise details and elegant finishing of the ancient Roman workmanship can thus still be admired in the Pantheon. Also Michelangelo appreciated the qualities of this stone, which he used for most of his sculptures.

Each of the four main zones of the interior (the floor, the first level as far as the first cornice, the attic level from the first to the second cornice, and the ceiling of the dome), was originally laid out and decorated according to a subtly different scheme.

This complex contrast and sophisticated discordance between the interior decorative zones was not fully appreciated or understood in later centuries, and in fact the attic level was radically modified in 1753, being replaced by a monotonous scheme in Neoclassical style, with simple square panels framed by ornamental mouldings alternating with window-like recesses topped by pediments.

Although the ancient materials were sadly lost the original scheme was reproduced in a small section of the south west wall in the 1930s. It consisted of oblong gaps like small windows with three panels like little pilasters on either side, standing above a broad horizontal band of marble.

The above-mentioned relationship between the circle and the square, which underlies the basic geometry of the entire building, is also mirrored by the floor decorations that still maintain the original design (although much of the marble has either been replaced or relaid). This consists of a checker-board pattern using grey granite, red porphyry and white or yellow marble. Inside the large squares are circles which form diagonal rows in a subtle contrast to the major north-south axis of the building.