Ancient Crosses Images percolating through the plaster - Aya Sofya (Istanbul)
When the Byzantine Church was converted into a mosque in 1453, christian images got plastered and islamic motifs painted over. But now, after 600 years, you can see the old Byzantine crosses images percolating through the plaster.
For more information about Aya Sofya, read en.wikipedia.org/wiki/Hagia_Sophia.