A series of earthquakes weakened the eastern arch and main dome of the Hagia Sophia, Justinian's monumental cathedral completed only two decades earlier, and the dome came crashing down onto the ambo and altar. Justinian summoned Isidore the Younger, nephew of one of the original architects, to design a replacement. The new dome, raised higher and ribbed for strength, has substantially survived to the present day.
When the Hagia Sophia was first completed in 537, contemporaries marveled that its vast dome seemed to float on air, suspended as if by a golden chain from heaven. Justinian is said to have entered and declared, 'Solomon, I have outdone thee.' But the original dome was flatter and more ambitious than engineering of the era could comfortably sustain, and cracks had already appeared after earlier tremors.
On this day a major earthquake struck Constantinople, and the compromised structure gave way. The eastern semi-dome and the main dome crashed down, destroying the ambo, altar, and ciborium beneath. It was a staggering setback for an empire that had poured enormous wealth and prestige into the building as a symbol of Justinian's imperial and religious authority.
Justinian did not hesitate. He commissioned Isidorus the Younger, nephew of original co-architect Isidore of Miletus, to rebuild. The new dome rose about twenty feet higher than its predecessor, with steeper curvature and reinforced ribbing that distributed stress more effectively. Completed in 562, this second dome has endured for over fourteen centuries, surviving further earthquakes, becoming the enduring symbol first of Byzantine Christianity, then Ottoman Islam, and now a museum straddling both legacies.