Digging under is my problem.
We build our pens a minimum of 60'x 60' or 3,600 square feet. Before sinking the t posts, we mark out our perimeter then use a small trencher to cut about a 6" deep trench around the entire perimeter. This makes sinking the t-posts easier and, once done, the bottom of you panels will be about 3-4" deep into the soil. Never had anything dig under this set-up. For pre-existing pens, you might try affixing a Predator Barrier along the bottoms of your pens to deter the digging under.