Tourists visit the Hagia Sophia in Istanbul on May 18, 2013. On International Museum Day, several national museums of Turkey were free in a limited time for the tourists and public to visit. Hagia Sophia is one of the most visited museums and most prominent monuments in the world in terms of art and the history of architecture. It was used as a church for 916 years and following the conquest of Istanbul by Fatih Sultan Mehmed, the Hagia Sophia was converted into mosque. [Xinhua/Lu Zhe]