Hagia Sophia, Istanbul, Turkey — Emperor Justinian I's masterpiece served as a cathedral for 916 years, a mosque for 482 years, and a museum for 86 years before being reconverted to a mosque in 2020. Its 31-meter dome was the largest in the world for nearly a thousand years and remains a supreme achievement of Byzantine engineering.
© Photograph: Myrabella, Public domain, via Wikimedia Commons
