The Pantheon, dedicated to 'all the gods,' replaced an earlier temple destroyed by fire. Hadrian's version, completed around 126 AD, featured a dome 43 meters in diameter with a central oculus open to the sky, achieved through graduated concrete mixtures that grew lighter with height. Its engineering remained unsurpassed for nearly 1,300 years and it survives today as the best-preserved major building of ancient Rome.
Roman engineers solved the dome's structural problem by varying the density of the concrete itself, using heavy travertine aggregate at the base and progressively lighter pumice near the top, around the oculus, a 9-meter-wide opening that lets rain fall onto the marble floor below and lets a shaft of sunlight sweep across the interior like a sundial through the day.
Hadrian, notoriously, did not inscribe his own name on the facade; instead he retained the original dedication by Marcus Agrippa from the earlier Augustan-era temple, a rare act of imperial modesty, or perhaps calculated humility as propaganda. The result is one of history's great architectural riddles, a building whose true builder is announced nowhere on its front.
Converted into a Christian church in 609 AD, the Pantheon escaped the fate of stripping and ruin that befell other Roman monuments. Its dome inspired Brunelleschi's Florence cathedral and Michelangelo's design for St. Peter's Basilica, and it remains, after nearly nineteen centuries, in continuous use, still holding the record among unreinforced concrete domes.