Hagia Sophia started off as an important Christian church before becoming a mosque and now is a UNESCO World Heritage Site (Museum). It was build in AD 537, during the reign of Justinian, seen above in the painting on the ceiling of the dome in the building. It is one of the most visited sites in Istanbul.
During its time, it was the world’s largest building and an engineering marvel of its time. It is considered the epitome of Byzantine architecture and is said to have “changed the history of architecture.
In 1453, Constantinople was conquered by the Ottoman Empire under Mehmed the Conqueror, who ordered this main church of Eastern Orthodox Christianity converted into a mosque. The bells, altar, iconostasis, and other relics were destroyed and the most mosaics were eventually destroyed or plastered over. Islamic features – such as the mihrab and four minarets were added.
It remained a mosque until 1931 and then it was re-opened in 1935 as a museum.
There is a lot of controversy surrounding the status of Hagia Sofia, which is also known as Aya Sofia. There is a hearing currently in Turkey on changing the status from a museum to a mosque. The ruling will be made in a few days (check the BBC link below)