The Hagia Sophia is a mosque and major cultural and historical site. It started as a Greek Orthodox church then became a mosque when the ottoman empire conquered it. Later it became a museum. More recently it once again became a mosque. For almost a millennium after its construction, it was the largest cathedral in all of Christendom.  It served as a center of religious, political, and artistic life for the Byzantine world and has provided us with many useful scholarly insights into the period.