Distinguished by its size and rising succession of domes, the Hagia Sophia is the supreme masterpiece of Byzantine architecture. First a church, later a mosque, and now a museum, it was designed under Justinian I by Anthemius of Tralles and Isidorus of Miletus and completed in less than six years. Its combination of a longitudinal basilica with a main building was wholly original in the sixth century, as was its use of pendentives. The Hagia Sophia stands on the site of what earlier structure? Discuss