== Description ==
Hagia Sophia is covered by a central dome with a diameter of 31 meters (102 feet), slightly smaller than the Pantheon's. The dome seems rendered weightless by the unbroken arcade of arched windows under it, which help flood the colorful interior with light. The dome is carried on pendentives: four concave triangular sections of masonry which solve the problem of setting the circular base of a dome on a rectangular base. In Hagia Sophia the weight of the dome passes through the pendentives to four massive piers at the corners. Between them the dome seems to float upon four great arches.
At the western (entrance) and eastern (liturgical) ends, the arched openings are extended by half domes carried on smaller semidomed exedras. Thus a hierarchy of dome-headed elements build up to create a vast oblong interior crowned by the main dome, a sequence unexampled in antiquity.
The structure has been severely damaged several times by earthquakes. The dome collapsed after an earthquake in 558; its replacement fell in 563. There were additional partial collapses in 989 and 1346.
All interior surfaces are sheathed with polychrome marbles, green and white with purple porphyry and gold mosaics, encrusted upon the brick. On the exterior, simple stuccoed walls reveal the clarity of massed vaults and domes.
== Later history ==
