A whole village of little black dirt hills appeared in my yard
seemingly overnight. There must be 20-30 of them in one 10x12
area, most about three inches in diameter. There are similar
but smaller villages elsewhere in the yard.

I scraped some of the dirt hills away and found a small hole,
maybe 3/8" in diameter, under each one. It looks like the
hole of a burrowing snake or something.

What have I got? Is this thing harmful to the lawn, or does
its tunneling help aerate the ground? The areas it has chosen
to tunnel in are only sparsely covered with grass, so it
really can't do much damage unless it spreads to the healthier
areas.

Any help is appreciated. Thanks.