The original cupola collapsed entirely after the earthquake of 558; in 563 a new dome was built by Isidore the younger, a nephew of Isidore of Miletus. Unlike the original, this included 40 ribs and was raised 20 feet, in order to lower the lateral forces on the church walls. A larger section of the second dome collapsed as well, in two episodes, so that today only two sections of the present dome, in the north and south side, still date from the 562 reconstructions.