Hagia Sophia in Istanbul was commissioned by Emperor Justinian I and completed in 537.
The building replaced two earlier churches on the same site in Constantinople. The architects Anthemius of Tralles and Isidore of Miletus created a vast domed structure that became the principal cathedral of the Byzantine Empire.
Hagia Sophia served as an Eastern Orthodox cathedral until the Ottoman conquest of Constantinople in 1453, when it became a mosque. It was opened as a museum in 1935 and was reconverted into a mosque in 2020.
Its history explains why it is sometimes described as a former cathedral rather than a present-day cathedral. The building’s dome, mosaics, marble decoration, and long religious history have made it one of the world’s most influential monuments.