My Seven Wonders of the World, #6
Hagia Sophia and its millennia-younger imitator the Blue Mosque in Istanbul, Turkey. (The nine hundred years newer Blue Mosque is prettier from the outside and its dome cleaner and brighter inside, but the older church set the standard.)
As architectural achievements the domes of these buildings are incredible feats. As a sensory experience they are stupefying. Both boast amazing open space under their domes. Seeming acres of open floor stretch out with no columns intervening. Photographs do not capture the feeling of immensity.
Both give the illusion of being larger inside than they seem from outside. I have noticed this phenomena with similar structures, including large (now vanished) circus tents and Cold War hardened aircraft shelters. From the outside the vault or dome slopes away from the viewer, making it seem smaller than it is. Inside, the viewer experiences the full immensity of the covered space. For that reason, Saint Peters Basilica and gothic cathedrals may be larger but lack the feeling of immensity.
A unique experience.