Look very close.
What do you see?
If you said ants, you are right!
I've been battling these little buggers all summer. The "hills" appear overnight, but a quick dousing of Ant-B-Gone Dust takes care of them until a week or two later when another batch or should I say "hatch" appears in a different location.
They are the tiniest ants (less than 3/8 of an inch) I have ever seen and in the thickest concentrations.
I first noticed a hill in my yard about five years ago. The hill was nearly three inches tall and was all ants, no dirt.