The inscription on the Pantheon’s portico credits Marcus Agrippa, but his temple on the Campus Martius burned in 80 AD, Domitian’s rebuild burned in 110 AD, and the domed building standing today went up under Trajan and Hadrian between 114 and 126 AD. Hadrian kept Agrippa’s name to signal continuity with Augustus, a move that fooled even Cassius Dio. Inside, the floor to the oculus and the width of the rotunda both measure 43.3 meters, so a perfect sphere fits within. The 4,500-ton dome stands with no steel reinforcement thanks to pozzolana mortar and aggregate that shifts from heavy travertine at the base to terracotta, tufa, and finally floating pumice near the 9.1 meter oculus, all over 6.4 meter walls hiding chambers and relieving arches.

Out front, 16 granite columns of 60 tons each were cut at Mons Claudianus in Egypt, dragged across the desert, floated down the Nile on the spring flood, shipped to Ostia, and hauled up the Tiber. A ghost roofline above the porch suggests 50 Roman foot columns were planned; only 40 foot shafts arrived, a mismatch studied by Mark Wilson Jones and Rabun Taylor. In 609 AD the Byzantine emperor Phocas gave the building to Pope Boniface IV, which saved it as a church. Constans II stripped its gilded bronze roof tiles in 663, Pope Urban VIII melted the portico’s bronze trusses mostly into cannons for Castel Sant’Angelo, and twin bell towers mocked as the ass’s ears stood until the late 19th century. Raphael and Italy’s first kings are buried inside.

• Why Livy’s rule of one god per temple, for lightning strikes, makes the name Pantheon likely a nickname
• Modern analysis showing uniform concrete would have put 80 percent more stress on the dome’s arches
• The oculus as a reverse sundial and the sloped floor whose ancient drains still carry away the rain
• The Roman pasquinade: what the barbarians did not do, the Barberini did
• Brunelleschi’s Florence dome and Jefferson’s University of Virginia rotunda as descendants of the design