Won't it be impossible for any Pokemon to dig without touching the ground? Think about it. Do you REALLY think that it is possible to dig a hole, and go into that hole without at some point, touching the ground, or being at a point where what affects the ground directly affects you?
Of course, unless you're implying that Pokemon detaches it's spiritual essence and that bit goes underground, while the Pokemon's true body remain above the ground and the spiritual essence strikes the opponent's spiritual essence for damage that translates to actual HP damage.
Ofc, I have no idea what I just said there either.