The Hagia Sophia is a former Greek Orthodox Christian patriarchal basilica, later an imperial mosque, and now a museum in Istanbul, Turkey. Built in 537 AD, it was the largest cathedral in the world for nearly a thousand years, until the completion of the Seville Cathedral in 15
From Wikipedia
Hagia Sophia mosque in İznik (Nicaea) in Bursa Province, Turkey, was built as a Byzantine-era basilican church. Converted into the Orhan Mosque after the Ottoman conquest, it was turned into a museum in 1935. The church is now once again in service as a mosque. It is in the town centre of İznik, within the old walled area.