Answer: Interesting! I'm not sure what was tunnelling under your lawn but it's probably gone now since the area is more exposed now that the snow has melted. Since you have sod you may be able to lift the sod and fill in the burrow with fresh soil and then relay the sod. The sod should knit together and hide the damage within a few weeks. I'd use a shovel to dig through the sod on one side of the tunnel and pull back the sod to fill in the sunken areas and then set the sod over and step down on it so the roots make good contact with the soil.
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