The Hagia Sophia was built in AD 360 as a Byzantine Christian church and has undergone many transformations during its time. It was converted to a Greek Orthodox Church in 1054, and a Roman Catholic Cathedral in 1204. It has been destroyed and rebuilt many times due to fires and earthquakes.
In 1453, when the Ottoman Empire conquered Istanbul, the Hagia Sophia was converted into a mosque. The cathedral used to be adorned with mosaics of Jesus Christ and other religious figures. As these figures were forbidden in Islamic mosques, these mosaics were hidden under tiles during the conversion. Luckily, this preserved the mosaics, which can now be seen in all their glory.
In 1935 the Hagia Sophia was converted into a museum, becoming one of the most significant monuments not only in Turkey but the entire world. It is no surprise that it became a UNESCO World Heritage Site in 1985.