Learn Architecture Online on Instagram: "The Pantheon — Rome’s 2,000-Year-Old Engineering Masterpiece Built nearly two millennia ago during the reign of Augustus, the Pantheon remains one of ancient Rome’s greatest architectural achievements. Its monumental dome — still the world’s largest unreinforced concrete dome — features a central oculus that brings light, air, and even rain into the temple. But what seems like a flaw is actually a stroke of Roman engineering genius: The marble floor, form