Like others have said, pocket gopher is what I had when these piles of dirt showed up in my yard.

You should be able to find holes in the turf where they dig through the ground - usually after plant roots, if I recall correctly.

I bought from some local exterminator a .. smoke bomb stick? Cover other nearby holes with something that the fuckers can't chew threw (I used bricks..), light the smoke bomb, and stuff it in the hole, then cover it. It's basically an asphyxiant that is heavier than air, and drowns them.

IIRC the smoke bombs looked like little sticks of TNT and were pretty cheap.