The home of the Norwegian National Opera and Ballet, in Oslo, resembles a glacier that’s calving great wedges of glass and white marble, and at night its windows, which are huge, project sheets of amber light onto the Oslo Fjord. Yet the Opera’s main impact, since its opening, in 2008, has been civic rather than aesthetic. It was built on an old industrial site as part of a larger effort to reclaim a stretch of ruined waterfront, and, despite its unpromising location, its roof—which slants upward from the harbor and seems to emerge from the water—has become a busy public square. Parents push baby carriages to the top; tourists pull suitcases from the train station; swimmers, sunbathers, kayakers, and swans treat the western edge as a beach. Dog walking, Tai Chi, and sunset watching are popular. For a performance of “Carmen” in 2009, the opera company showed a free simulcast on a large screen in front of the building, and some five thousand people spread picnic blankets on the roof to watch it. During the building’s inaugural performance, a young couple were discovered making love above the auditorium. One of the architects told me that he considered their act both a compliment and the building’s “consummation.”