The circular museum designed by Frank Lloyd Wright is overflowing with almost 100 creations gathered in a seemingly surreal scavenger hunt. White neon tubes impale a raincoat draped on a seven-foot bundle of twigs. A red motorcycle with antlers dangles from the spiral ramp, 50 feet above the main floor. The ramp itself sprouts igloos covered with broken glass, bread, wax, twigs or rubber. Outside, big neon numerals multiply up the facade to a stuffed alligator.

The man who put this gallimaufry together, with 20 aides over a month's time, stands on the ramp, gazing downward. "This," says Mario Merz, "is important."

At 64, Merz is one of Italy's most esteemed artists, and the retrospective at New York City's Guggenheim Museum is his greatest accolade: The show, running till Nov. 26, makes him the first artist to fill the 30-year-old museum top to bottom with his works. "Merz has helped forge a new vocabulary—neon, glass, stone," says Thomas Krens, the Guggenheim's newly appointed director, but precisely what the vocabulary means is tough to say. "Is so strange, this kind of work," Merz admits. "Is not easy to understand. Is something very complex, like a dream."

Born in Turin, where his father designed engines for Fiat and his mother taught piano, Merz quit med school for art and in 1945 was arrested for distributing anti-Fascist literature. "Was necessary to draw in prison for pass the time," he says. "I draw on the very little small piece of paper [in which] my mother send me il formaggio, the cheese." After the war he began experimenting, using sprays, industrial enamels and neon. He was entranced by mixing materials to show "natural light and industrial light in same work, or history and the future."

Merz and his artist-wife, Marisa, share a Turin studio, and he still cares deeply about his impact on the world beyond its walls. "When people look freely," he says, contemplating a rubbery igloo, "maybe this work helps them to see in a new way."