Normally, boundaries are only two-dimensional, and as far as I know
everything straight below and above the surface belongs to the
country that IS that surface. I don't know of any lower or upper
limitations of national soil (centre of the earth??) or airspace
(outermost atmosphere??), but I think the boundaries "up" and "down"
are at right-angles with those on the surface of the earth (of course
not when the boundary is on a slope, but I think you know what I
mean). But in a book I recently bought on a second-hand book market,
I found this item about the benl boundary in the river Meuse:

"The sovereignty of both riverine states extends to the thalweg; at
bridges, however, the boundary is in the middle of the central arch
of the bridge, a point which doesn't coincide with the thalweg below.
An invisible line from the thalweg below the bridge to the point in
the middle of the central arch of the bridge is the imaginary
boundary line. This imaginary line practically never is at right-
angles with the water of the Meuse. This can only be if the thalweg
is right in the middle, which practically never is the case. The
imaginary three-dimensional line is at a constantly shifting angle
with both river and bridge, as the thalweg is constantly shifting."
(from "De grens gemarkeerd, Grenspalen en grenskantoren aan de
landzijde", by Paul Spapens and Kees van Kemenade, Hapert, 1992, page
34)

Are there any other examples of oblique (vis-a-vis the vertical)
boundary lines? Perhaps also below the (ground or water) surface?

Any ideas, anyone?

Peter S.