Hagia Sophia is a former Greek Orthodox cathedral, later an Ottoman imperial mosque and now a museum located in Istanbul, Turkey. It was built by Justinian I in 537 AD and was the world's largest building and an engineering marvel of its time. The cathedral was later converted into an Ottoman mosque from 1453 AD until 1931 AD. It was then secularized and opened as a museum in 1935 AD. Hagia Sophia was inscribed as a UNESCO World Heritage Site in 1985 as part of Historic Areas of Istanbul.