The impossible cube or irrational cube is an impossible object that draws upon the ambiguity present in a Necker cube!
impossible cube illusion, viewed from a certain angle, this cube appears to defy the laws of geometry.
An impossible cube is usually rendered as a Necker cube in which the edges are apparently solid beams. This apparent solidity gives the impossible cube greater visual ambiguity than the Necker cube, which is less likely to be perceived as an impossible object. [source]
irrational cube illusion, viewed from another angle
BTW, a doctored photograph purporting to be of an impossible cube was published in the June 1966 issue of Scientific American, where it was called a Freemish Crate.
