Well, I've had experience a bit with such things...first, the
suggestion posted later on about running an reinforcing wire along the
bottom of the cyclone fence, woven through the bottom holes, is
good..but if they start diggin' it's not gonna work. But, if you want
to try that then any Ace Hardware, Lowe's, Home Depot, or other home
improvement/hardware store should have something that'll help..but you
may need a fence stretcher like they use with barbed wire to make it
stiff enough to really help.
However, they do make a low voltage electric fence system for small
pests (and small pets) that, supposedly, isn't as rough on smaller
animals as the stuff they use to rein in cattle, and I've purchased one
from Tractor Supply Company for a friend to keep his Boston Terrier
from digging under their fence and it worked beautifully. Mark the
electric fence with ribbons every few feet (3 or 4) just to be
sure..but I think they'll find it anyway and power it up. It took only
once for the Boston Terrier to find it and remember it. We put it low
to the ground as the fence above was suffient to stop the dog.
The dog learned quick to stay away from the fence and that was the
point. At that point she pretty much quit shocking herself.
All you bleeding hearts who want to flame me about using an electric
fence for a dog can un-hitch your wagons right now: 1) it was low
voltage for small animals, and 2) it's better than the animal getting
hurt or lost or in a fight because it's too clever to be held by a
conventional fence. Besides, after a few interactions with the fence
the dog figured it out and pretty much quit trying it (I can't swear to
that because I don't live there).