Hagia Sophia Grand Mosque, located in Istanbul, Turkey, is a stunning architectural masterpiece originally built as a cathedral in the 6th century by the Byzantine Emperor Justinian I. It was converted into a mosque in 1453, later into a museum in the 20th century, and was re-designated as a mosque in 2020. It is known for its massive domed roof, beautiful mosaics, and rich history that spans religions and empires.