Read this first! Start out by placing the top edge of the image below at the top edge of your monitor screen. It's a closeup of the surface of the Saturnian moon Tethys, as imaged by the Cassini-Huygens spacecraft. Note the heavily-cratered surface. Slowly scroll down, admiring the craters, and notice that at some point you will start seeing what appear to be boulders on a rough surface, rather than craters. If you now scroll back up, what looked like craters before now also look like boulders. Most Web presentations of this “crater/hill” illusion don't really work, for most viewers, because of poor design or concept. This real-life example works excellently. Try it!  Can you cause the flip from crater to boulder to occur by an act of will?  Try it!

Illusion 1, Illusion 25, Illusion 50, Illusion 75, Illusion 110, Illusion 120, Illusion 130, Illusion 140, Illusion 150, Illusion 160, Illusion 170, Illusion 180