Anthemius, the Greek geometrician who designed the dome, wanted it to appear as if it were suspended from heaven on golden chains. So he used columns that supported arches filled with glass windows to create the illusion that the dome is floating. (You squint when you look up into the light, which makes the columns disappear.) Note the seraphim (winged angels). Two still have their faces covered by golden masks installed by the Ottomans, but the third’s mask was removed during a 2010 restoration.