Germans strolling down the genteel Treppenstrasse in Kassel, Germany, in late June of 1972, might have been surprised to see an unexpected addition poking out of the facade of their beloved Fridericianum. One of the world’s first independent public museums and a model of classical style and enlightened culture built in the late 18th century, the institution once employed brothers Jacob and Wilhelm Grimm in the library. In 1972, it was in the midst of hosting the Documenta contemporary art show, and as part of the event, a group of experimental architects were installing a plastic orb amid the columns and stonework on the facade, a pneumatic pimple of PVC foil, 8 meters in diameter, that contained a catwalk of tubular steel. Created by…