First a church, then a mosque, and now a museum. In 1453 the church was converted into a mosque following the fall of Constantinople. All of the alters and sacrificial vessels were removed and the mosaics depicting Jesus, Mary and others were either removed or plastered over. In 1935 the mosque became a museum and since, many of the mosaics have been uncovered. Restoration is still in progress as you can see the scaffolds in many of these photos.
This is just another of the many sights in Istanbul that makes it, for me, one of the most magical and mystical places I have visited.