Voles do that here. I had a large piece of plywood down last year for a while and when I eventually lifted it I found tunnels and hole all over underneath it from voles digging. I've only actually seen one vole in our yard but I know they are around. Their population seems to stay low though.I lifted the plastic in the garden, and I am still traumatized half an hour later. The worm was eighteen feet (okay, inches) long, and as wide as my pinky!!
This is what the rodents were doing under the plastic.
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