As its name suggests, the Pantheon in Rome was dedicated to many gods. Its seven interior niches housed statues of Apollo, Diana, Mars, Mercury, Jupiter, Venus, and Saturn, and the great dome also had religious significance since it symbolized the heavens. Even now, when stripped of much of its enrichment, the scale and simple geometry of the Pantheon awe the visitor. Moreover, its sophisticated engineering stirs imagination for its ancient engineers. Many of their modern counterparts are at a loss to understand how the structural system worked, much less how it has survived for two millennia. The great Florentine artist Michelangelo Buonarotti concluded that it was the result of angelic and not human design.
The first Pantheon was built in 27 b.c. for Marcus Vipsanius Agrippa, son-in-law of Augustus Caesar. Except for lower parts of the porch and foundation, it was irretrievably damaged by fire in a.d. 80, and reconstruction to a slightly different design was commissioned by the Emperor Hadrian. The present building dates from between 118 and 128, although some scholars believe that it was completed under Pius about twelve years later. Lucius Septimius Severus and Caracalla sponsored a restoration in 202. In 609 the building was presented to Pope Boniface IV by the Byzantine emperor Phocas, and it was dedicated as the Church of St. Mary of the Martyrs (now Santa Maria Rotonda) on 13 May.The great portico was originally approached across a colonnaded rectangular courtyard on the temples north side. Three rows of 46-foot-high (14-meter) columns support its gable roof, which rises to 80 feet (24 meters). The entablature carries a pediment that once was crowded with bronze relief sculptures of a battle between the Titans and the gods. The front row of eight columns and the second row of four are of Egyptian gray granite the third row, framing the door and its flanking apses, is of Egyptian red granite. All are crowned with Corinthian capitals carved in white Pentelic marble from Greece. The massive bronze entrance doors are 21 feet (6.3 meters) high. With their fanlights, they were originally gold plated.
The interior of the Pantheon a single volume is a 143-foot-diameter (43.3-meter) cylinder upon which rests a hemispherical dome. The total inside height is the same as the diameter. A semicircular apse covered by a hemidome faces the door. Around the wall on each side are three 14-foot-deep (4.2-meter) recesses, alternately rectangular and semicircular in plan and screened from the central space by pairs of 35-foot-high (10.5-meter) marble Corinthian columns supporting entablatures and a deep cornice. The lower part of the wall once was faced with marble and porphyry, the upper with marble pilasters and paneled with antique yellow marble, serpentine, and pavonazetto.
Above it all soars the magnificent coffered dome, the largest in the world until Brunelleschi built his masterpiece on Florence Cathedral thirteen centuries later. Five rows of twenty-eight square diminishing coffers, each recessed in four steps, rise to a central oculus a circular window open to the sky. Originally the coffers were decorated, perhaps with gold stars on a background of blue. Even when the doors are closed, light enters the vast space through the 19-foot-diameter (8.7-meter) oculus. As the sun moves, a spotlight slowly swings across the interior, illuminating and enriching the colored wall and floor surfaces. Externally the dome was covered with gilded bronze plates, but they were taken to Constantinople in 655 and replaced by lead. Indeed, much of the precious material of the Pantheon has been plundered the exterior surface of the wall was once veneered with colored marbles. Now the brickwork and the huge relieving arches of the second tier are exposed.
A 14.75-foot-deep (4.5-meter) concrete foundation supports the Pantheons 20-foot-thick (6-meter) cylindrical brick-faced concrete wall, 104 feet (31.7 meters) high. Roman concrete consisted of lime, pozzolana and a few pieces of very coarse aggregate. Bricks of various shapes were used as lost formwork. The wall was made lighter as it rose by using lighter aggregate in the concrete every 4 feet (1.2 meters) the brick formwork was tied together with a through-course of brickwork. It was designed to bear a range of complex stresses and loads, and it seems that instead of working as a solid mass it behaves structurally like a series of massive piers acting as buttresses to resist the thrust imposed by the dome.
Roman concrete domes normally were made of lightweight aggregates such as pumice to reduce their mass and constructed on timber centering supported from the ground. The thickness of the Pantheon dome reduces from nearly 20 feet (6 meters) at the base to about 5 feet (1.5 meters) at the edge of the oculus. The engineers probably knew that such a huge span of unreinforced concrete might not develop enough tensile strength therefore they used lighter aggregates near the apex and the oculus really a brick-reinforced compression ring to minimize lateral thrust. Thus, to halfway up the dome is built as a series of seven stepped rings of concrete with alternating layers of bricks and tufa probably, each was allowed to develop full strength before the next was placed. The top 30-foot (9.2-meter) section was made in the usual way: alternate layers of 9-inch (7.5-centimeter) pieces of tufa and volcanic slag bonded with mortar. Although the Pantheon did not employ a revolutionary structural system, it represents the high point in Roman concrete technology evolved from well-thought-through construction traditions.