For the past month or so, I have found a dozen or so small (~12" round, 2-3" high) dirt piles on the lawn. At first I thought a small critter was digging up grubs but the grass is intact below the dirt, or at least looks like it is...just looks like someone piled a couple of shovels of dirt on the lawn. I also thought that it might be bees making a ground nest. I have had some sod bees this year but there is usually an opening for the underground nest. Any ideas, short of small alien crop circles, as to what might be doing this?