Penrose-Treppe
The Penrose stairs is an impossible object created by Lionel Penrose and his son Roger Penrose. It can be seen as a variation on the Penrose triangle. It is a two-dimensional depiction of a staircase in which the stairs make four 90-degree turns as they ascend or descend yet form a continuous loop, so that a person could climb them forever and never get any higher. This is clearly impossible in three dimensions, unless each of the steps is sloped; the two-dimensional figure achieves this paradox by distorting perspective.
The best known example of Penrose stairs appears in the lithograph Ascending and Descending by M. C. Escher, where it is incorporated into a monastery where several monks ascend and descend the endless staircase.
The staircase had also been discovered previously by the Swedish artist Oscar Reutersvärd, but neither Penrose nor Escher were aware of his designs.
In terms of sound, the Shepard tone is a similar illusion.
In popular culture
Christopher Nolan incorporates Penrose staircases into his film Inception, since it is explained that normally-impossible structures can be created exist within lucid dream worlds.