The Hagia Sophia (Saint Sophia or Holy Wisdom) was built as an Eastern Orthodox church in 537 in Istanbul (then Constantinople, capital of the Byzantine Empire). It was built on the site of two earlier churches that had burned down. It served the Orthodox faith until 1453, when the Turks sacked Constantinople. (It was briefly a Roman Catholic church from 1204-1261 when the Latins occupied Constantinople.) After the sack of Constantinople, it was converted into a mosque, and remained so until 1935. As a mosque, it had a significant effect on Islamic architecture; the other major mosques in Istanbul, such as the nearby Blue Mosque, all closely resemble the Hagia Sophia. In 1935, Ataturk made the building into a museum.
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The dome itself is an example of Byzantine contributions to architecture, specifically the pendentives that support the dome as it transitions from a square base atop the building to its own circular base, which also gives the dome its famous "floating" illusion. The dome has collapsed several times because of earthquakes and is currently undergoing some restoration, as you can see in the photo.
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