The nave of Hagia Sophia as seen from the South Gallery. The entrance is to the viewer's left. The floors and walls are entirely covered in marble, while the upper domes and galleries are covered in mosaics or bright paint. On the right is one of the four massive pillars that supports Hagia Sophia's great dome. Straight ahead on the ground floor is the south aisle and, behind the gated windows, the Library of Mahmut I. Also visible are two of eight calligraphic roundels that have hung in the Hagia Sophia since the 19th century. They each bear an Arabic name; the one on the right is one of the four caliphs of early Islam; the one on the left is one of the Prophet Muhammad's grandsons. In the foreground is a large marble urn, one of two near the entrance of the Hagia Sophia.