I have a couple diggers that come over on a regular basis and I have found that the two biggest reasons to dig a hole at least in my yard when we're outside playing is either to get cool and those are normally holes the size of a body. LOL Or what I call ankle breakers and those holes are digging for moles or whatever animal is there, was there, or imagined that was there. ( and those normally come from terriers breeds the great hunters) In my experience as long as the dogs are kept occupied and other stimulation The digging out of boredom is almost nonexistent. Unless of course you have that one dog that digging is their favorite past time and then I find that making sure they're well supervised and have lots of interaction that can normally be only a small concern.