An Optical Illusion:

Once again I found a way to play a joke on myself. The lap pool I swim in has a white bottom with a column of small blue tiles in the middle of each lane. When I swim the crawl stroke, my head is usually under water, and those tiles give me a sense of my speed. The pool is mostly three feet deep but at one end the depth drops to five feet, and there, I always seem to slow down – the tiles move past my eyes more slowly. I could not understand how the depth of the water could affect my speed, and I tried many changes in my swimming to make sense of this.

Finally I determined that I'm not slowing down at all. When the depth increases, the same blue tiles are farther from my eyes, and therefore they seem to go past me more slowly. You can see the same effect by sweeping your eyes across any tiles, first up close and then several feet away.