Hi Lesley, this optical illusion depends on which direction you are looking at it. If you rotate the picture (or your screen) by 180 degree, the illusion will disappear. I was also amazed the first time I saw it on the beach. If you are interested, you can have a look at one of my older posts here https://365project.org/etienne/365/2019-01-22 or have a better explanation here https://www.moillusions.com/embossed-footprints-illusion/