Click on the link above, then go to downloads. Click on an MPEG and fly through an Escher painting in 3-D!
I've seen explanations of Escher's work that typically say something like:
"Escher's work looks just fine and reasonable because you see a certain projection of 3-D on to 2-D. But that projection is what makes the impossible possible. When you look at the actually 3-D structure needed to give that oh so special projection, well, that's where the problem comes in."
See the 3-D structure now.