Enlarge (credit: Maksym Kozlenko / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)) The 1,500-year-old Hagia Sophia in Istanbul, Turkey, will become an active mosque beginning on July 24, ending its 85-year run as a secular museum. Byzantine Emperor Justinian I ordered the building's construction in 532 CE; for nearly 1,000 years, its 55.6 meter (180 ft) dome covered the largest indoor space in the world.