[edevinney]

They are wily and destructive little bastiges. There is definitely a second opening to that hole, and filling it up without closing the other end is a waste of time. Groundhogs do hibernate, but won't do so for a month or two. They have very good senses and can be hard to get a sight on, but do come out in the morning and evening to browse for greens to eat. A 12ga or AK would be satisfying but overkill - pistol-caliber shots hell's and CB long .22 both work very well. The CBs are pretty quiet from a rifle-length barrel, too.

I have tried filling the hole with the body of the recently-deceased occupant. It was satisfying but ineffective: next year I found a groundhog skull in the dirt pushed out by the new owner of the hole...