Technically, Hagia Sophia [wiki] (Greek for the Church of the Holy Wisdom of God) is no longer a church, it is now a museum in Istanbul, Turkey. It began its life as an early Christian church, then rebuilt as the seat of the Eastern Orthodox Church in Constantinople, then a mosque when the city fell to the Turks in 1453 before it finally became a museum.
Hagia Sophia as we know it today was completed by Byzantine Emperor Justinian I in 537. When completed, the temple was so large and richly decorated that Justinian proclaimed “Solomon, I have surpassed thee!”. It remained the largest church for one thousand years after it was completed.
Hagia Sophia is one of the greatest surviving examples of Byzantine architecture, with a large central dome and interior intricately decorated with mosaics, marbles, and stone inlays. The dome, often referred to as the vault of heaven, was a new architectural feature at the time, necessitating the invention of a new pillar support system.
Today, the restoration of Hagia Sophia is a delicate balance of restoring Christian iconographic mosaics under historic Islamic art, which would have to be destroyed to reveal the work underneath.