The floors are marble, worn smooth from human footsteps, billions of them made across one thousand four hundred years by worshipers and unbelieving visitors alike. Unconscious minutes pass while you orient yourself, then at once you recognize the enormity of the structure you have just entered. Under a massive dome space unrolls and widens around you, decompresses you like deep-breathing — in a building originally a church, then a mosque, then a museum and now a mosque once again. It has survived multiple earthquakes and military sieges. As religious constructions go, it is second in magnitude only to the temple complex of Karnak at Luxor, Egypt.