The Church of the Holy Saviour in Chora was originally built outside of the walls of Contantinople (now Istanbul) in the 5th century. It was originally a monastery. The church has been rebuilt a few times, once because of an earthquake. Gorgeous mosaics and frescos cover the walls and ceilings, each one telling a different story. In 1511 the church was converted into a mosque and the mosaics and frescos were covered with a layer of plaster. In 1945 it was converted into a museum and the mosaics were then uncovered. And they were absolutely beautiful. You can learn more about Chora Church here.