Hagia Sophia: Cathedral, Mosque, Museum, Mosque

Hagia Sophia, the Church of the Holy Wisdom, was completed in 537 CE under the Byzantine emperor Justinian I as the cathedral of Constantinople, replacing two earlier churches on the same site that had each been destroyed by riot and fire. Designed by the mathematician Anthemius of Tralles and the architect Isidore of Miletus, its enormous dome, carried on four massive pendentives over a nearly square nave, created the largest enclosed interior space in the world at the time, a rank the building held for close to a thousand years and an engineering achievement still studied today.

Sultan Mehmed II converted the building into a mosque within days of the Ottoman conquest of Constantinople in 1453, adding minarets, a mihrab oriented toward Mecca, and eventually the great calligraphic roundels bearing the names of Allah, Muhammad and the early caliphs that still hang inside. The Turkish Republic's founding government secularized the building in 1934 and opened it as a museum the following year, a status that held for eighty five years and made Hagia Sophia one of the most visited cultural sites in the world, its Byzantine mosaics and Ottoman Islamic elements displayed together as a single layered monument to both traditions.

In July 2020, Turkey's Council of State annulled the 1934 secularization and President Erdogan reclassified the building as an active mosque, a decision that took effect within weeks and returned regular Muslim prayer to the space for the first time since 1935, covering rather than removing the building's most prominent Christian mosaics during prayer times.