I had the same problem with my dog going either under or over the fence (6 foot). I'm pretty sure he was just bored, getting him fixed didn't help. Getting picked up by the pound didn't help just cost me money. The answer, go to Home Depot get and electric fence. Run the wire about 6 inches off the ground. Guaranteed to stop the digging and only cost about $45. It's a whole lot easier than buring posts, if he's going over the fence run two wires around the top both ground and positive. The reason for this is when they are up on the fence they are not grounded good enough for the wire to work, run buth wires close together and you'll hear a satisfying yelp, so you can rest assured your mutt is in the fence and not out getting hit by a car. 
