The "church of holy wisdom", Haghia Sophia is among the world's greatest architectural achievements.
More than 1,400 years old, it stands as a testament to the sophistication of the 6th-century Byzantine capital.
Following the foundation of Constantinople in AD 324, the vast edifice was built over two earlier churches and inaugurated by Emperor Justinian in AD 537.
In the 15th century the Ottomans converted it into a mosque and now it is being used as a museum.