Hagia Sophia, Istanbul

Completed in 537 AD under Emperor Justinian I, the Hagia Sophia's 31-metre central dome appeared to Byzantine observers to be "suspended from heaven by a golden chain." Its engineering — pendentives supporting a dome that seems to float weightlessly — was so revolutionary that it took Western architecture 1,000 years to catch up. It served as a church for 916 years and a mosque for 482 years before becoming a mosque again in 2020.