What keeps the wolf's from digging out under fence to get out

They bury the fence 3 feet down into the ground. They've found the wolves won't dig that deep.

We have a 3' long ground skirt attached to the main vertical fence, which lays on the ground (it’s like an “L” shape). The ground skirt is then covered with dirt/wood chips. (wolves will definitely dig deeper than three feet - one of the many reasons it is so difficult to attempt to contain one in a yard!)