Reasons your dog may be digging:

Boredom

If dogs are left outside or confined for long periods of time without proper stimulation, a dog can suffer from a lack of physical and psychological outlet.  This can cause an array of beharvioral problems such as digging and destructiveness.  Exercise, games, toys, training and play are essential.

Cooling

If you find your dog is digging a hole to lie in, the it may be hot.  Be sure your dog has an adequate shelter away from the sun.  You may consider supplying it with a child's wading pool to cool to it off.  A floating ball in the pool and the right motivation can also provide a fun and entertaining new summertime game.

Rodents

Is you dog digging after a rodent or other animal in your yard?  Contacting an exterminator or wildlife removal may be necessary.

Escapte

If your dog is attempting to excape, there are many options depending on the type of enclosure or fencing.  Check with a dog trainer for a varitety of solutions.

Digging for the Love of It

If you are the proud owner of a Terrier or any dog that just loves to dig, give them a safe palce to expel their drives.

Construct a sand box.  4-6' x 6' is a great size.  Supply the box with dirt combined with playground sand.  Attract your dog to the box by burying toys, balls and treats in the sand.  Encourage your dog to "find" the treasures.  Praise your dog for finding the objects.  Eventually bury the toys deeper in the box.

Carefully watch your dog.  Anytime you see him beginning to dig outside of the box, quickly tell him "NAAH!" and redirect him toward the box commanding him, "DIG IN THE BOX."  Continure until the behavior is extablished.