The Church of the Divine Wisdom (Hagia Sophia in Greek) in Sultanahmet, Istanbul (map), is one of the most impressive and important buildings ever constructed.
Its wide, flat dome was a daring engineering feat in the 6th century, and architects still marvel at the building's many innovations.
Called Hagia Sophia in Greek, Sancta Sophia in Latin, Ayasofya in Turkish, it was built on the site of Byzantium's acropolis (map) by Emperor Justinian (527-65 AD) in 537 AD.Ayasofya was the greatest church in Christendom, and was meant to be. According to Prof. Robert Osterhout, it was built to surpass the gigantic Church of St Polyeuchtos erected by Julia Anitzia, scion of the line of Theodosian emperors.
Julia meant her church, a "recreation" of the Temple of Jerusalem, to symbolize her wealth, power and legitimate claim to the throne of Byzantium. Justinian had to out-build her to establish his own legitimacy—and he did.
His church remained the largest church ever built until St Peter's Basilica was constructed in Rome a thousand years later. (Julia's church, by the way, was destroyed by an earthquake. You can see a few pitiful ruins of it near the traffic under/overpass between the Istanbul Belediye Sarayı [City Hall] and Aqueduct of Valens [Bozdoğan Kemeri](map).
Being the world's most impressive building, it's no wonder that Mehmet the Conqueror proclaimed it a mosque soon after his conquest of the city from the Byzantines in 1453.
It served as Istanbul's most revered mosque until 1935 when Atatürk, recognizing its world-historical significance, had it proclaimed a museum, as it is now.
Although most of the building is still a museum, a room on the east side was opened in 2007 as a prayer-place (Ibadete Açık Kısmı), and the call to prayer is proclaimed from the minaret above it.
Ayasofya is awe-inspiring—one of the first things to see when you're in Istanbul. Luckily, it's right next to Topkapı Palace, the Blue Mosque and the Byzantine Hippodrome, and right across the street from Yerebatan, the Sunken Palace Cistern.
The 30 million gold tesserae (tiny mosaic tiles) which cover the church's interior—especially the dome—are now being restored to the brilliance they boasted 1500 years ago. The interior was filled with scaffolding for more than 17 years, but as of February 2010, much of the scaffolding has been removed, so if you visited before when the scaffolding was in place, you may want to visit again. There's more to see, and it looks far better, since therestoration.