Guggenheim Museum, Bilbao

You really have to see this building to believe it — no photograph has ever done justice to this symphony of shapes, so alive that they seem ready to take wing. American architect Frank Gehry used blocks of limestone and undulating sheets of titanium to turn the notion of modern architecture on its ear. So thoroughly did he succeed that two new terms were born from it: "The Bilbao Effect" — the ability of a city to turn its fortunes around by constructing a single world-class building — and "architourism," a whole segment of the travel industry revolving around landmarks of contemporary architecture. Inside the museum are traveling exhibitions and rotating displays of its own collections of modern art.