When you arrive in Istanbul, you search for the cathedral. It must be this old Orthodox cathedral-turned-mosque-turned-museum, known as what?

The Byzantine structure, Hagia Sophia, has stood in Istanbul since 537 AD. As the world's largest building at the time, it was likely known far and wide, its renown even reaching Scandinavia. In fact, there is purportedly Viking graffiti in the Hagia Sophia, with some Norseman named Halfdan leaving his signature inside. In its time, it served as an Eastern Orthodox cathedral, a Roman Catholic cathedral, and an Ottoman mosque. The glorious dome is now a museum.

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