As said for 'Another World', the zenith and nadir, are only relative concepts: in this case, where the 'down' stops, the 'up' starts. This is a vertical cylinder vr: the only wrong effect is that when you 'drag the mouse' left or right, the image pans up or down, so a self-moving track was added. See the original.

"No one of us needs to doubt the existence of an unreal, subjective space. All our senses reveal only a subjective world to us; all we can do is think and possibly mean that therefore we can conclude the existence of an objective world."
M.C.Escher
From a letter to J.W.Wgenaar