We keep our pet rabbits in a portable exercise pen (sort of like a covered toddler play yard) in a wild area of our garden. They are great at keeping the weeds under control. On a couple of occasions however, they have dug into a gopher hole and escaped under pen's fencing. I'm thinking about creating a more permanent home for them in the garden but I wonder how deep I would have to install the fence underground so they wouldn't dig under. Alternatively I could install cinder block underground along the perimeter of the pen. Does anyone have any experience with this? Thanks!