Though it looks like stone, the Bulgarian Orthodox Church of St. Stephen of the Bulgars (Sveti Stefan) on the Golden Hornin Balat (map), is made of cast iron.
It was was cast in Vienna, floated down the Danube on 100 barges, and bolted together here in Istanbul in 1871.
In December 2017, seven years of extensive restorations were completed. Ninety percent of the church was affected. In 2018 the church should be open to services and visits.