Hagia Sophia was apparently filled with a collection of holy relics, including the shroud of Mary, nails from the true cross and the tombstone of Jesus, until the Fourth Crusades ransacked the city in 1204. The Crusaders also replaced the Patriarch with their own Latin Bishop,which is thought to have cemented the Great Schism between Greek Orthodox and Roman Catholic. But the weakened Byzantines fought back and claimed it once again in 1261. By the time Sultan Mehmet the Conqueror stormed the city in 1453, their strength failed them and they lost their church as it was converted into an imperial mosque. The stone cannonballs that litter the gardens of the outer courtyard today are said to be the very cannonballs that Mehmet the Conqueror used to sack the city. It served as the principal mosque of Istanbul for almost 500 years.
Hagia Sophia’s sealed underground chambers fed the minds of mystical daydreamers. Legends talked of an imprisoned devil and the possibilities of the Byzantine’s attempt to protect sacred relics. There was even thought to be a secret passage that extends from Tekfur Palace, next to the city walls, to the islands of the Marmara Sea. Regardless of myths, Turkish divers recently searched the flooded chambers. The floor directly under the Hagia Sophia’s magnificent dome covered a main water reservoir that was 12m deep. Their discovery found two tunnels that were possibly used by the 5th-century Byzantine Emperor Theodosius II to privately go to Tekfur Palace and the hippodrome.