[FreeInWV]

Cover all holes but one. Light a fuzee (road flare), drop it down that hole and cover it up. It will suck every bit of oxygen out of the length of the hole. It will work.

Somewhere there is a video of the verminator. It is a device that allows you to fill the hole with propane and ignite it. Looks like lots of fun.

[Othniel]

Custodian at school had this as his preferred method of dealing with gophers, so I'm betting he'd try the same with groundhogs. Only he'd use multiple flares.