This is a variation on M.C.Escher’s 1960 lithograph Ascending and Descending.  At first we see the staircase leading into the distance, and ending in mid-air, but then at about 15 seconds in, the furthest point of the staircase  lines up with the nearest point.  At the same time the perspective cues that made the distant platform a bit smaller, and a bit paler, than the nearest platform vanish so that they appear the same size.  Our brains then assume that they connect up, even though the result is a stair that could not exist in real 3D space. From 20 seconds in, the penguins entering from the right are always heading towards us in an endless loop, but never get any nearer.

This post is an update on a much earlier one, from 2008.