Rome has been called the city of domes. The greatest of them sits majestically atop St. Peter’s Basilica, the largest and grandest church in the world. By law, no building in Rome’s city center may exceed the height of St. Peter’s. Recognized as a UNESCO World Heritage Site in 1984, it is a massive church in the style of the Italian High Renaissance located in Vatican City, an independent microstate within Rome, Italy. It contains countless treasures, antiquities, and art, and truly takes one’s breath away, both in its epic scale and artistic splendor.
Its most impressive feature is its centerpiece: the great dome, soaring 448 ft high (137m) and which dominates the skyline of Rome. The dome marks the tomb of St. Peter, the first Pope and disciple of Jesus, and was built over the papal main altar and final resting place of the Blessed Apostle. Designed byMichelangelo Buonarroti when he assumed work on the project in 1546, it was only finished in 1590, long after the great master had died. It is the tallest dome in the world, and a true engineering marvel. It has inspired countless domes across the world and draws millions of visitors a year. Let us explore seven secrets of this dizzying dome.
Interior views of the mosaic-decorated dome of St. Peter’s Basilica (by author).
1. There was an Old St. Peter’s, and it didn’t even have a dome:
The church we now call St. Peter’s Basilica is the “New” St. Peter’s, built between 1506 and 1626. The previous church that existed on the same location is now referred to as “Old” St. Peter’s, and it existed from the 4th to the 15th centuries. It was built by Constantine on the location of the ancient Roman Circus of Nero and Caligula (1st century CE), where St. Peter had been killed and buried.
Architect and polymath Leon Battista Alberti assessed the old basilica in the 15th century:
“I have noticed in the basilica of St. Peter’s in Rome a crass feature: an extremely long and high wall has been constructed over a continuous series of openings, with no curves to give it strength, and no buttresses to lend it support... As a result, the continual force of the wind has already displaced the wall more than six feet (1.8 m) from the vertical; I have no doubt that eventually some slight movement will make it collapse...”
Then, after eleven centuries of continuous use and various attempts to restore it, the building was deemed unsalvageable. The project of replacing it was begun by the ambitious Pope Julius II in the beginning of the 16th century. He wanted it razed to the ground, shocking everyone. This was the original church in Rome, built over the tomb of the first Pope, St. Peter, who was killed in 64 CE – how could he think of purposefully destroying it? However, the future won out and the old church was slowly demolished. Only a few pieces were ultimately preserved in the new church.
Interestingly, the Old St. Peter’s church did not have a dome. It was modeled on the old Roman basilica design, which did not have a central dome but a five-aisled, Latin cross-plan. It was capable of accommodating 3,000-4,000 worshipers, with five aisles, a wide central nave and two smaller aisles to each side, which were each divided by 21 marble columns.
In 1503, the famous architect Donato Bramante won the competition held by Pope Julius II to design the new St Peter’s Basilica. He commissioned Bramante, as the new capo maestro (“head master”) to replace the Old St. Peter’s with a church so magnificent it would dwarf the monuments of ancient Rome. Bramante originally wanted a Greek-cross design for the Church (with sides of equal-length) topped by a shallow concrete dome like the Pantheon. He reportedly said: “I shall place the Pantheon on top of the Basilica of Constantine.” Unfortunately, Pope Julius died in 1513, Bramante in 1514, and Raphael took over for a relatively unproductive six years.
A) the first design for the dome of St. Peter’s by Bramante, B) another design for the new St. Peter’s by Antonio da Sangallo the Younger, ultimately rejected by Michelangelo. This design, later discarded, was made into a huge wooden model now kept at the Vatican Museums. (Left; Donato Bramante/Public domain Right; Public domain)
2. It might have looked completely different, as suggested by a huge wooden model:
St. Peter’s may have looked completely different, as evidenced by a large intricate wooden model still preserved in the Vatican Museums. When Bramante died in 1514, just one year after Julius II, his vision of a St. Peter’s with a low Pantheon-like dome was never realized. When he passed away, only the four main crossing piers of the new Basilica were in place, and four arches spanned between them. Then disaster struck: Rome was sacked in 1527 and work at the church ground to a halt (the perpetrators were troops of Emperor Charles V).
The architect Antonio da Sangallo the Younger finally took over in 1536. His main practical contribution was to strengthen Bramante’s piers, which had already begun to crack. He also wanted to include two high bell towers flanking the central dome. These ideas were made manifest in a huge wooden model, 1:30 scale, which is still preserved in the offices of the Fabbrica di San Pietro. The massive, detailed wooden model was commissioned by Pope Paul III, constructed from 1536–1539, cost 4,000 crowns (about half a million dollars today), and took approximately seven years to create.
Saint Peter’s Basilica, final form cross-section and floorplan, by Sangallo, showing the more pointed dome of Della Porta. From: Fletcher, Banister (1946) A History of Architecture on the Comparative Method (17th Ed.). (Public Domain)
Gerriann Brower notes: “Sangallo’s elaborate, costly wooden model is unrecognizable as the St. Peter’s we know today. Michelangelo intensely disliked his ideas. Besides being public enemies, as a result from other competing projects, Michelangelo alleged that Sangallo’s design would promote crime with all its niches and side chapels – he said there were enough dark areas to ensure nuns would be impregnated. Sangallo disrespected Michelangelo for lack of building experience. Once again, when Sangallo died suddenly in 1546, all work ceased on St. Peter’s.”
Left: Michelangelo’s designs for St. Peter’s, engravings by Etienne du Perac, 1569, from “Speculum Romanae Magnificentiae” (The Mirror of Roman Magnificence). It was rendered by Étienne du Pérac in 1569 and shows Michelangelo’s more hemispherical design around the time of the great artist’s death. Now in the MET. Right: Michelangelo, Studies (in black chalk) for the dome of St. Peter’s, Rome, circa 1550-60. Now in the Teylers Museum, Netherlands. The double dome and internal stairwell are clearly visible. (Public Domain)
3. Michelangelo designed it, but didn’t see it completed:
On January 1, 1547, at age 71, Michelangelo was appointed the new capo maestro of St. Peter’s by Pope Paul III. After a lifetime creating some of the greatest art in history, his greatest project still lay ahead. He focused on designing the dome, the crowning achievement of the church. No one had ever dared make one so large. He modified Bramante’s original design, and removed Sangallo’s additions, such as the two bell towers. His new design called for a double-shelled dome of concrete masonry, travertine, and protective lead plates, while the inside would be covered in stucco and mosaic. The double-shell design with an inner cavity would reduce weight by 40%. A few original drawings showing these features still exist from the hand of the great capo maestro. They demonstrate how Michelangelo constantly redesigned his structures as he went, experimenting with outer dome designs of differing dimensions and degrees of steepness.
Sadly, after having designed the dome, he only managed to complete the drum of the dome before his death at age 88 in 1564, after which work again ground to a halt. Gerriann Brower observes: “The last 17 years of Michelangelo’s life were devoted to St. Peter’s. He didn’t do it for the money or prestige. It was his divine salvation. His design and construction of the monumental dome stands out as one of his often-overlooked masterpieces that has influenced other buildings for centuries to come.” The interrupted state of the Basilica that was left upon his death can be seen on paintings and engravings, such as from the 1585 coronation of Pope Sixtus V.
A view of St. Peter’s after Michelangelo’s death, but before the dome was raised. Giovanni Battista de’Cavalieri. The Jubilee in 1575, with a View of the old Church of St. Peter’s. Engraving with etching. 1575.
It would take another twenty-six years before the dome was finished. The architects Giacomo della Porta and Domenico Fontana, the new capo maestri, finally raised the dome in just twenty-two months between 1588 and 1590, thanks to the tireless work of over 800 laborers. The outer shell was made of lead plates, which were hoisted into place using pulleys and scaffolding. Interestingly, Della Porta decided to keep Michelangelo’s inner dome design but elevated the outer dome, giving it a more pointed profile, similar to the Duomo, while simultaneously reducing lateral stress.
As the 16th century drew to a close, the finishing touches were put on the great dome. Fontana completed the lantern and had an inscription to the honor of Sixtus V placed around the dome’s inner opening. The lantern allows natural light to flood the interior of the church. Finally, in 1593 Clement VIII had the great silver cross placed on top of the lantern, an event that was accompanied by the ringing of all of Rome’s church bells. Hidden inside the arms of the silver cross are two lead caskets for relics, one containing a fragment of the True Cross and a relic of St. Andrew and the other containing silver medallions of the Holy Lamb.
According to William E. Wallace in his 2019 Michelangelo, God’s Architect: The Story of His Final Years and Greatest Masterpiece,
“Given than Michelangelo did not control some 88% of the building’s history, it is astonishing that he accomplished as much as he did and that we give him credit for the church. We consider St. Peter’s to be his building – not Bramante’s, even less Raphael’s, and certainly not Giuliano or Antonio da Sangallo’s. Giacomo della Porta and Carlo Maderno may have completed the church, but no one credits them as its author … it was less important for Michelangelo to build a specific design than to protect the integrity of the church. It was God’s church and he was God’s architect.”
Montage of views of the Pantheon, the great Roman temple and concrete dome constructed by Hadrian between 118 and 125 CE. It served as inspiration for the dome of St. Peter’s. (By author).
4. It was inspired by famous domes, and inspired others:
According to John Paul Sonnen, “Michelangelo designed the dome to be the largest in the world. It was inspired by the dome of the Pantheon in Rome and that of the Duomo of Florence that was designed by Brunelleschi a century earlier in 1436.” Brunelleschi’s earlier dome for the Duomo, the Cathedral of Santa Maria del Fiore, had been built in Florence between 1420 and 1436, and served as the primary inspiration for Michelangelo’s even grander dome.
The most important innovation Michelangelo drew from Brunelleschi was his concept of two shells—a lighter inner structural shell and an outer protective shell, essential for stabilizing a dome without relying on external buttresses. He also adapted the technique developed by Brunelleschi of ribbed masonry, but instead of dividing his dome by eight ribs like in Florence, he used sixteen.
Michelangelo admired the Florentine masterpiece so much that he designed a similar, yet technically more advanced, structure. He famously said “I build a greater, but not more beautiful, dome” as he copied and improved upon every element of the earlier Florentine church. He, and Della Porta after him, doubled the number of ribs, used a double helix herringbone brickwork pattern instead of the earlier single pattern, improved the materials, set the dome on a higher vertical drum, and made a bigger lantern to admit light into the church.
Comparing the dome of St. Peter’s (left, by author) with that of another dome that directly inspired Michelangelo, the Duomo in Florence (right), his hometown. (by Gary Campbell-Hall/CC BY 2.0)
According to R.A. Scotti in her 2007 Basilica: The Splendor and the Scandal: Building St. Peter’s: “Della Porta formed the shells almost entirely of heavy masonry laid in a herringbone pattern. Frequently used by the architects of ancient Rome, the method for fitting bricks together in an inverted V design applied pressure equally from both side, preventing hoop tension and collapse.”
The mathematics behind this construction technique were laid out in a 2020 article in Engineering Structures by researchers at Princeton University and the University of Bergamo. They observed: “The cross-herringbone spiraling pattern allows the construction of self-balanced domes.” While the Duomo dome used a single spiraling pattern of vertical bricks, St. Peter’s used a double spiraling pattern, a cross-herringbone pattern called a “double loxodrome”.
The dome of St. Peter’s has undoubtedly impacted architecture around the world. The most famous dome in America, Washington’s Capitol Building, in addition to the central dome of the Berlin Cathedral, Mary Queen of the World Cathedral in Montreal, and Sir Christopher Wren’s design for the new St. Paul’s Cathedral in London: they were all directly inspired by the dome of St. Peter’s.
St. Paul’s Cathedral in London, exterior and interior views of the great dome built by Sir Christopher Wren, who was inspired by St. Peter’s (by author).
5. There is a secret room in the bronze ball at the summit:
Most people who have visited Rome will know that a cross sits atop St Peter’s. Much fewer will know that under this cross is a metal ball over 2.5m in diameter. Called the Palla di bronzo (“bronze ball”) it is made of 54 trapezoidal-shape bronze plates. Completed on July 20, 1593, the sphere was the final step, the crowning touch of the dome. In 2005, 410 years later, it was discovered during routine cleaning that the bronze ball was actually covered in gold.
Even more fascinating: it contains a secret room that up to 16 people can access through an 80cm opening at the top of a ladder. This secret room greatly appealed to past aristocracy, as immortalized by more than 70 marble tablets hung inside the church. Each tablet commemorates the visit of a sovereign or a royal family member to the sphere, such as king Ferdinand of Naples, Prince Gustaf of Sweden and Norway, and Tsar Nicholas I of Russia I in 1845.
Exterior detail of St. Peter’s from the roof, showing the Palla golden ball (by author).
Hear how the American traveler William M. Gillespie, who visited Rome in 1844, described his visit:
“… We had to go up yet another ramp that runs around the lantern and finally we went to the top, at the foot of the stem that supports the sphere and the cross. This stem is hollow and contains a ladder, perpendicular, on which we climbed and, through a narrow opening, struggled into the sphere. It is eight feet in diameter and the sphere seemed to swing back and forth and give way with every gust of wind. Instinctively we feared that the thin copper plate of which it is made could break or that our weight could make it sway and fall from the superb height, and then bounce off the lantern on the dome of the roof, until it crashed to the ground with its precious contents, to four hundred and thirty feet from the point of departure. We didn’t stay there long…”
The last pope to climb into the ball was Pius IX, one evening in 1847. Afterwards, he and his cardinals settled down for a picnic on the roof of the church. Today visitors can likewise lunch on limonata, cornetti, and gelato from the roof-top café. Unfortunately, access to the globe was forbidden to the public a few years later for safety and conservation reasons. Since then, only the staff in charge of maintaining the basilica can go inside.
6. It is held together by huge iron rings that weigh up to 18,000 pounds:
Despite Della Porta having installed three supporting iron rings within the masonry as he erected the dome, it still began cracking forty years later. This was due to its construction speed, as well as its foundation: poor, marshy Vatican soil. An earthquake in 1703 did not help.
This situation is described by Wayne Kalayjian in his 2024 Saving Michelangelo’s Dome: How Three Mathematicians and a Pope Sparked an Architectural Revolution:
“When Pope Benedict XIV sent a team of three mathematicians to inspect the dome in 1742, they found, as Benedict suspected they might, a cataclysm waiting to happen: 350 feet above ground level, the colossal structure was riddled with cracks, many clearly visible from the floor of the basilica…”
These three men were Thomas Le Seur, François Jacquier and Roger Joseph Boscovich, and they were commissioned by the Pope to write a report on the status of the dome. Called The Opinion of Three Mathematicians on the Damage Found in the Dome of St. Peter's (The Parere for short), it was a landmark report in the history of engineering. It detailed mathematically how the installation of five further iron rings around the dome would stop the cracks and secure it for the future.
According to Kalayjian, “The concepts expressed inside their succinct, thirty-six page Parere had thunderous implications. It demonstrated that the simpler days of the master builder and capomaestro were long gone and that the more intuitive times of Vitruvius, Brunelleschi, and Michelangelo were over. These three unassuming mathematicians had invented the future.”
Sadly, their cutting-edge suggestions were ridiculed by the Pope’s master builders: Venetian physicist Giovanni Poleni and Roman architect Luigi Vanvitelli. Ironically, despite mocking the mathematics of the Parere (which likely went over their heads), they oddly enough still followed its suggestions. They wrapped the dome in five, and finally six, massive new iron hoops, locking the dome tightly into place. “The cracks had not returned, and Michelangelo’s dome had been seemingly stabilized and spared from further damage and potential catastrophe.” (Kalayjian).
Meanwhile, master builder and self-taught genius Nicola Zabaglia assisted this process with his revolutionary new “hanging” scaffolding, which was so good it was only replaced in the 1940’s with tubular steel. Zabaglia even wrote a book describing his inventions.
Zabaglia’s scaffolds for the restoration works of the Vatican Obelisk and his innovative “hanging” scaffolds inside St. Peter’s Dome used to install the iron rings. (N. Zabaglia, Castelli e Ponti, 1824, pl. XXVI).
Finally, in 1748, six years and six hoops later, the job was done. These huge rings still silently hold together Rome’s tallest building and iconic symbol of Christendom, buried deep inside, so it may last another 400 years.
The locations of the six iron rings added to stabilize the cracking dome 1744-48, from the base to the lantern of Saint Peter’s dome. The order of placement is indicated by letters (the lower-case letters show the position of the former rings). (G. Poleni, Memorie istoriche della gran cupola del tempio vaticano, Stamperia del Seminario, Padova, 1748).
7. You can climb to the top through a winding stairway between its two domes:
Fortunately visitors can still access the top of the Basilica, for some of the best views of ancient and modern Rome. The access winds its way through the two titanic domes, and visitors will have to slant their bodies inward as they spiral upwards. The climb is 551 steps, or 320 if you take the elevator, and there are three levels of the dome you can visit.
Montage of views climbing and descending the 551 stairs between the two domes, including the prize view from the top of the entire city (by author).
So if you find yourself in Rome in 2026, you will find joyous celebrations at St. Peter’s Basilica celebrating the 400th anniversary of its dedication. While you won’t be able to visit the Palla’s secret room, you can still climb the 551 stairs to the top of St. Peter’s famous dome for the best panoramic views of the Eternal City. And you can even get a gelato and pizza in the newly expanded rooftop restaurant and admire the dome up close and personal.
Top image: Left; Sunset behind the mighty dome of St. Peter’s Basilica, Vatican City. Right; The author with the dome of St. Peter’s taken from the Vatican Gardens. Source: Jonathon A. Perrin
Jonathon Perrin is an author and explorer. He has written five books on Amazon. He was fortunate enough to travel around Italy for a month in 2025 exploring everything historical. He is currently writing a book about his Italian adventures called Italy in a Year of Hope: Adventures in Rome and the Amalfi Coast. It is due out this summer.
Visit www.jonathonperrin.com for more.
References
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