While in Lisbon two weeks ago, I was struck by this illusion of waves in Rossio Square (which many others have noted, of course). Even when standing on the flat surface, things appear undulating if you look away from your feet.
What I also find as interesting, when reviewing my photos is the impression that the walking family appears to have been added from another image. That being my ‘explanation’ for the illusion. They were not. Seeing is not always believing.