The border is not just a fence line. There is always a neutral zone between the posts, which can be traced, not to mention the border guards who inspect the fences every day. Therefore, we can say with confidence that you will dig an eight-kilometer tunnel longer than it will take any visa examination and obtaining a foreign passport.
I don’t know about the tunnel, but our guys swam across the Urals and ended up in Kazakhstan. And those who from Kazakhstan, on the contrary, swam across to us in the Orenburg region. True, we have a single economic zone with Kazakhstan, but the guys are unlikely to even take their passport with them into the river (a civilian is needed, a foreign one is not needed).