Untitled (1964) is a crystalline form made of steel and mirrored plastic, which reflects all the surfaces of the surrounding architecture and reflecting itself within its illusionistic interior. In a related drawing from 1964 titled Three Works in Metal and Plastic, Smithson notes that with this sculpture creates a "Double vanishing point—exists as a solid reversal of traditional illusionistic perspective. Infinity without space." He also writes that in the angled mirrored surfaces at the center of the form the "floor is reflected over ceiling" and that "the entire room that the piece is in, is absorbed by the piece."

With this sculpture Smithson is manipulating the relationship between sculpture and architecture, upending expectations of traditional spatial relationships and playing with illusions of infinity through mirrors. This large crystalline wall sculpture is made of steel, paint, and polymethyl methacrylate (commonly referred to as acrylic or the name brand Plexiglas—in the drawing Smithson calls this material "glowing rose mirrors also called 'neon' plastic").

Robert Smithson's Untitled (1964) is in the collection of the Museo Reina Sofia, in Madrid, Spain.