The Pantheon has stood roofed and whole for nearly nineteen centuries, and its dome remains the largest ever built in unreinforced concrete. Emperor Hadrian raised it around AD 125, yet kept the name of Agrippa—builder of the first Pantheon—carved across the portico. The builders made the concrete lighter as it climbs—heavy travertine at the base, porous pumice near the crown—so the shell seems to float. At its centre a single round opening, the oculus, is left forever open to the sky. The building survived because in the seventh century it became a church, sparing it the quarrying that reduced most of ancient Rome to rubble.
Step inside and the scale rearranges itself. The interior is a perfect sphere, exactly as tall as it is wide, and the whole room turns around the shaft of light falling from above. That beam slides across the coffered ceiling and marble walls through the day like a slow sundial. Rain falls straight in and drains through small holes worn into the original Roman floor. Today the Pantheon is both monument and working church; the painter Raphael lies buried against one wall, beside two kings of Italy.