What would be the best way to render a large terrain either up close OR alternatively zoomed out to it's extents and half of it hidden to show a cross section?

Development World Creation
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I’d like to have a rather large (20x10km) chunk of standard terrain with the ability to zoom in and out on it, RTS style.
When completely zoomed out I want to be ablet to switch to a cross-section view of that terrain, showing some subterranean features.
This cross-section is pre-defined, the cutting line doesn’t have to be dynamic.

Importing the terrain as a landscape works great for the first part but how will I be able to clip or dissolve half of it for the zoomed out view?
As I see it for the cross-section it would be better to have it as one object?
Should I something like a mode switch where on zooming out I switch to a different scene?