The Hagia Sophia was originally completed in 537 A.D. by the Byzantine emperor Justinian 1. At the time, it boasted the largest dome, and the largest interior space, in the world. Most of the non-figurative mosaics in the building date from this early period. The figurative mosaics of that time were destroyed during the two Byzantine Iconoclasm periods in the 8th and 9th centuries A.D. New mosaics were added to the nave and dome, and the upper galleries, in the 10th and 12th centuries A.D.
But these newer works endured their own hardships. During the Fourth Crusade and the Sack of Constantinople in 1204, many were pried off the walls and shipped to Venice. And after the Ottoman conquest in 1453, the Hagia Sophia was converted to a mosque and most of the remaining mosaics were plastered over or otherwise defaced. Those that you can see today were uncovered during restoration efforts in the mid-ninteenth century and the early part of the 20th century.
The mosaics of the Hagia Sophia have suffered many indignities over the building’s long history, but those that remain are impressive, and well worth a good look while you are there. The detail and realism of the portraits, in particular in the Deesis mosaic, are truly spectacular.