Terriers are particularly likely to do this as it is very natural behaviour having been handed down in their genes from their working ancestors.  Sometimes terriers or other small dogs try to dig out other animals, such as rabbits, and get stuck down their holes, especially when out on a walk.  If your dog is likely to do this, make sure he wears a gps tracking device on his collar so he can be found and dug out again easily if he gets lost. 

To discourage a dog from digging for animals that can be heard underground, make sure he is getting enough predatory games with squeaky toys instead.  Teach him how to play these games and then always provide plenty of opportunity to do so, regularly buying new squeaky toys if you need to.  Sometimes toys on springy string such as the Whippit can be very useful for encouraging dogs with a predatory nature to play.  Watch your dog carefully when in the garden and, if he starts to become interested in animals below ground, listening with ears cocked ready to pounce and dig, be ready to offer a much more rewarding game with the toys instead.  The more your toy play resembles a mouse hunt, the more your dog will enjoy it and want to do that instead in of digging.  Once he has used up his energy budget for the day, he will be less interested in digging up small animals to chase or catch.  In addition, get rid of the small animals by removing any food sources they might be living on so they go elsewhere to live.

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