An illusion of perspective, most famously rendered by MauritsCorneliusEscher in "Ascending and Descending":
http://britton.disted.camosun.bc.ca/jbimpstair.htm
RogerPenrose and his father invented this and the tribar illusion and corresponded with Escher in the late 1950s.
See also Escher's page for an adaption in Lego!
http://www.andrewlipson.com/escher/ascending.html