The other day while searching for snail shells in fine gravel I had brought home from a beach, I noticed a white speck moving rapidly among the sand grains. It appeared smaller than the period at the end of a sentence. Despite its high speed, relative to its size, it went only a short distance, relative to my size, before it hid itself under a sand grain. That made it easy to relocate it by getting it to move again by touching a few grains.
Here are the pictures of it I took before I lost it for good.
I suspect it was a mite of some sort. Its body was about 0.25 mm long (the ruler division next to it was 1 mm long).