Hagia Sophia (meaning Holy Wisdom) is a famous Istanbul landmark. As a great architectural beauty, Hagia Sophia is an important monument both for Byzantine and for Ottoman Empires. Formal Greek Orthodox patriarchal cathedral, later an Ottoman imperial mosque and now a museum, Hagia Sophia was built in 537 AD as the largest building of that time. For almost a thousand years, Hagia Sophia was the largest Christian church in the known world. In 1935, the first Turkish President and founder of the Republic of Turkey, Mustafa Kemal Atatürk, has transformed the building into a museum. On July 1, 2016, Muslim prayers were held again in the Hagia Sophia for the first time in 85 years.