For nearly a thousand years, Hagia Sophia was the largest cathedral in the world, and stepping inside today it is immediately clear why it has captivated visitors across empires and centuries. Built in just five years by Emperor Justinian in 537 AD, this architectural marvel features a massive 56-meter-high dome that seems to float on a ring of light from 40 arched windows. The interior layers nearly 1,500 years of history — golden Byzantine mosaics depicting Christ and the Virgin Mary share the walls with enormous Ottoman calligraphy medallions and an ornate mihrab pointing toward Mecca. Now functioning again as a mosque, Hagia Sophia remains one of the most extraordinary buildings ever constructed by human hands.

Pro tip: Arrive 30 minutes before the first prayer of the day for the shortest lines, and look up at the Deesis mosaic on the upper gallery — it is considered one of the finest examples of Byzantine art in existence.