The Pantheon, the most perfect pagan building in the city, was built B.C. 27, by Marcus Agrippa, the bosom friend of Augustus Cæsar, and the second husband of his daughter Julia. The inscription, in huge letters, perfectly legible from beneath, “M. Agrippa, L. F. Cos. Tertium Fecit,” records its construction. Another inscription on the architrave, now almost illegible, records its restoration under Septimius Severus and his son Caracalla, c. 202, who, “Pantheum vetustate corruptum cum omni cultur restitverunt.” Some authorities have maintained that the Pantheon was originally only a vast hall in the baths of Agrippa, acknowledged remains of which exist at no great distance; but the name “Pantheum” was in use as early as A.D. 59.

In A.D. 399 the Pantheon was closed as a temple in obedience to a decree of the Emperor Honorius, and in 608 was consecrated as a Christian church by Pope Boniface IV., with the permission of the Emperor Phocas, under the title of Sta. Maria ad Martyres. To this dedication we owe the preservation of the main features of the building, though it has been terribly maltreated. In 663, the Emperor Constans, who had come to Rome with great pretence of devotion to its shrines and relics, and who only stayed there twelve days, did not scruple, in spite of its religious dedication to strip off the tiles of gilt bronze with which the roof was covered, and carry them off with him to Syracuse, where, upon his murder, a few years after, they fell into the hands of the Saracens. In 1087, it was used by the anti-pope Guibert as a fortress, whence he made incursions upon the lawful pope, Victor III., and his protector, the Countess Matilda. In 1101, another anti-pope, Sylvester IV., was elected here. Pope Martin V., after the return from Avignon, attempted the restoration of the Pantheon by clearing away the mass of miserable buildings in which it was encrusted, and his efforts were continued by Eugenius IV., but Urban VIII. (1623–1644), though he spent 15,000 scudi upon the Pantheon, and added the two ugly campaniles, called in derision “the asses’ ears,” of their architect, Bernini, did not hesitate to plunder the gilt bronze ceiling of the portico, 450,250 lbs. in weight, to make the baldachino of St. Peter’s, and cannons for the Castle of Saint Angelo. Benedict XIV. (1740–1758) further despoiled the building by tearing away all the precious marbles which lined the attic to ornament other buildings.

The Pantheon was not originally, as now, below the level of the piazza, but was approached by a flight of five steps. The portico, which is one hundred and ten feet long and forty-four feet deep, is supported by sixteen grand Corinthian columns of oriental granite, thirty-six feet in height. The ancient bronze doors remain. On either side are niches, once occupied by colossal statues of Augustus and Agrippa.

The Interior is a rotunda, 143 feet in diameter, covered by a dome. It is only lighted by an aperture in the centre, twenty-eight feet in diameter. Seven great niches around the walls once contained statues of different gods and goddesses, that of Jupiter being the central figure. All the surrounding columns are of giallo-antico, except four, which are of pavonazzetto, painted yellow. It is a proof of the great value and rarity of the giallo-antico, that it was always impossible to obtain more to complete the set.

Some antiquarians have supposed that the aperture at the top of the Pantheon was originally closed by a huge “Pigna,” or pine-cone of bronze, like that which crowned the summit of the mausoleum of Hadrian, and this belief has been encouraged by the name of a neighbouring church being S. Giovanni della Pigna.

The Pantheon has become the burial-place of painters, Raphael, Annibale Caracci, Taddeo Zucchero, Baldassare Peruzzi, Pierino del Vaga, and Giovanni da Udine, are all buried here.

The third chapel on the left contains the tomb of Raphael (born April 6, 1483; died April 6, 1520). From the pen of Cardinal Bembo is the epigram:

Taddeo Zucchero and Annibale Caracci are buried on either side of Raphael. Near the high altar is a monument to Cardinal Gonsalvi (1757–1824), the faithful secretary and minister of Pius VII., by Thorwaldsen. This, however, is only a cenotaph, marking the spot where his heart is preserved. His body rests with that of his beloved brother Andrew in the church of S. Marcello.

During the Middle Ages the Pope always officiated here on the day of Pentecost, when, in honour of the descent of the Holy Spirit, showers of white rose-leaves were continually sent down through the aperture during service.

In the Piazza della Rotunda is a small obelisk found in the Campus Martius. Following the Via della Rotunda from hence, in the third street on the left is the small semicircular ruin called, from a fancied resemblance to the favourite cake of the people, Arco di Ciambella. This is the only remaining fragment of the baths of Agrippa, unless the Pantheon itself was connected with them.

Behind the Pantheon is the Piazza della Minerva, where a small obelisk was erected in 1667 by Bernini, on the back of an elephant. It is exactly similar to the obelisk in front of the Pantheon, and they were both found near this site, where they formed part of the decorations of the Campus Martius. The hieroglyphics show that it dates from Hophres, a king of the 25th dynasty.