The Pantheon: Rome's Perfect Building
The Pantheon is the best-preserved ancient building in the world, and it is better preserved than most medieval buildings, because it has been in continuous use since its construction. Hadrian built it between approximately 118 and 128 AD on the site of earlier temples in the Campus Martius district of Rome, and it has served as a temple, a church, a tomb, and a tourist site across nineteen centuries without the structural interruption of abandonment. The dome that spans its interior has not been surpassed in diameter — 43.3 meters — by any unreinforced concrete construction in the two thousand years since it was poured. Whatever Rome’s engineers knew, they knew something that took a very long time to recover.
The building’s exterior presents a conventional temple facade: a portico of sixteen granite columns, a triangular pediment, and a broad flight of steps. This is, in architectural terms, a somewhat misleading introduction to what follows. The rotunda behind the portico — the circular drum topped by the dome — is something different from anything that preceded it, and the experience of moving from the relatively conventional exterior into the interior is one of the more striking architectural sequences in the Western tradition. The space inside the dome is a perfect sphere: the radius of the dome equals the height of the drum, so that a sphere 43.3 meters in diameter could be inscribed in the interior volume. The oculus — the open eye at the top of the dome, 8.8 meters in diameter — is the only light source, and it moves across the interior surfaces through the day as the sun moves across the sky. It also admits rain, which drains through the slightly convex floor.
The engineering that makes the dome possible is not fully understood even now. Roman concrete — opus caementicium — was made from volcanic ash called pozzolana, lime, seawater, and aggregate, poured in layers into wooden formwork and allowed to cure. The Pantheon’s dome was poured in concentric rings of decreasing density: the heaviest aggregate near the base, progressively lighter materials — pumice, empty clay pots — toward the oculus, reducing the structural load where the concrete needed to span the greatest distance. The coffered pattern of the interior ceiling serves a similar purpose: the coffers reduce the mass of the dome while the remaining ribs carry the load to the drum below.
The precise engineering of this load distribution is what has fascinated structural engineers since the Renaissance. The dome’s thrust is carried outward and downward to the drum by the geometry of the concrete mass; the drum transmits that thrust to the foundations through a system of brick relieving arches embedded in the concrete walls. The walls themselves are not uniform — they contain a series of alternating niches and chambers that reduce mass while maintaining structural integrity. None of this is visible from the interior, where the surface appears as undifferentiated concrete and marble facing. The engineering is hidden inside the walls, doing its work without announcing itself.
The building’s dedication is indicated by its name — pan theon, all the gods — but its precise religious function is uncertain. The niches in the interior walls presumably housed cult statues of major deities; the specific deities and their arrangement have not been recovered from the ancient record. Hadrian is reported to have used the building for judicial hearings as well as religious purposes, which suggests a civic as well as sacral function, though the combination would not have seemed unusual to Romans for whom the categories were not sharply distinguished.
Its conversion to a Christian church — Santa Maria ad Martyres — in 609 AD is what preserved it from the material stripping that destroyed most ancient Roman buildings. The bronze that had covered the original ceiling of the portico was removed — Bernini used it to cast the baldachin over the high altar in St. Peter’s, which produced the Roman saying quod non fecerunt barbari, fecerunt Barberini (what the barbarians did not do, the Barberini did, playing on Pope Urban VIII’s family name). The porphyry floor is largely original. The interior marble facing, though restored multiple times, substantially preserves the Roman decorative scheme. Raphael is buried there, as are two Italian kings.
Every subsequent dome in the Western architectural tradition is in conversation with the Pantheon. Brunelleschi studied it before designing the dome of Florence Cathedral. Michelangelo’s dome for St. Peter’s took it as both reference and challenge. The United States Capitol, the Panthéon in Paris, the National Gallery in Washington — the domed rotunda that has served as the architectural language of civic authority in Western culture for five centuries is Hadrian’s building, transmitted through the Renaissance and carried forward into every culture that has tried to express institutional permanence through built form. The oculus is still open to the sky. Rain still falls through it on certain days and drains through the ancient floor. The building that was built to house gods has been housing something ever since.