537 AD: The impossible dome
Completed under Emperor Justinian in 537 AD, Hagia Sophia (Holy Wisdom) was the largest enclosed space in the world for nearly a thousand years. Its dome appears to float on a ring of light — 40 windows around its base create the illusion that it is suspended from heaven. The Byzantine architect Anthemius of Tralles used mathematical principles not fully understood again until the 20th century.
1453: Conquest and conversion
When Mehmed II entered Constantinople after the 53-day siege, he rode directly to Hagia Sophia and converted it to a mosque on the spot. Christian mosaics were plastered over (which preserved them). Minarets were added — four remain. It served as the model for all subsequent Ottoman mosque architecture.
1934 to 2020: Museum, and after
Atatürk converted Hagia Sophia to a secular museum in 1934 — a symbol of the new Turkey's separation of religion and state. In 2020, it was reconverted to a mosque by President Erdoğan. The building is open to non-Muslim visitors outside prayer times; Christian mosaics are covered during prayers.