People tour the main room of the Hagia Sophia Museum in Istanbul, Turkey. Round signs containing Arabic calligraphy hang high on the walls. The original Hagia Sophia building, which was a church, was built by Constantine the Great in the fourth century. The current building, which is the third, was built between 532 and 537 A.D. and is a classic example of Byzantine architecture. The building was converted into a mosque in the mid-15th century. In 1934, the Hagia Sophia became a museum, and this is its function today.