Is that right? Wow, that's a very telling bit of trivia for all to contemplate, isn't it?
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I've been there too - Hagia Sophia is one of the masterpieces in the history of architecture, not just for it's beauty as an exemplar of the apex of Byzantine design, but for its use of the very pendentives (inverted triangular structural elements which transition between the support columns & a dome) where the plastered-over Seraphim mosaics are finally being exposed again.
There's a little more info on the mosaics here at HagiaSophia.com:
The 4 seraphims' faces were covered with 6-7 layers of plaster for almost 160 years during the sovereignty of Ottomans. The last person who saw the faces of the Seraphims was the Swiss architect Gaspare Fossati while he was holding the restoration at Hagia Sophia in 1840s. With a 10 day hard work, experts managed to take off the 7 layers of plasters and reveal the face of one of the seraphims....The [precise] age of the mosaics is unclear however they are known to be older than 700 years.