MI AC Parsons,
I don't know anything about Michigan, but we have hills similar to what you describe here in Tennessee. They are always in bottom land, near a creek. They are caused by crawfish from the creek.
I quit baling hay on a lady's field after last year. The crawfish mounds were so prevalent, I didn't want to risk my sickle bar mower, plus I had to run in one lower gear for raking and baling.
They're a real nuisance around here.
Tom In TN