Hagia Sophia is a former Orthodox patriarchal basilica, later a mosque, and now a museum in Istanbul, Turkey.
From the date of its dedication in 360 until 1453, it served as the Greek Patriarchal cathedral of Constantinople.
Famous in particular for its massive dome, it is considered the epitome of Byzantine architecture and is said to have "changed the history of architecture." It was the largest cathedral in the world for nearly a thousand years, until Seville Cathedral was completed in 1520.