Cable inference and mapping

Long time, no post. I just don't have the free time I used to .

Anyway... I sort-of caught up on my feeds and after reading this article about finding 'secret' government fiber lines, I started thinking. If the ground is very saturated with multiple crisscrossed cables, utilities, etc., is it reasonable to think that by looking at a map of those utilities and cables you can infer, with varying degrees of probability, the locations of these 'black' lines?

I would assume that in areas like the DC there are all sorts of classified communications lines going all over the place, so the logical place to start would be in the vicinity of a government building that has a high likelihood of having a classified or sensitive (non-commercial or public) line. Based on the locations of existing lines, one could extrapolate the rough areas where a line might leave the building and the path it takes (around the existing lines and utilities).

Maybe I'm thinking too two-dimensionally, but all this seems somewhat plausible. Let's say a 'black' line runs through an alley between buildings (unlikely but possible). Could someone dig through the basement to the line and tap it? I vaguely remember reading something about being able to tap a fiber line for around $100. The cost required to protect those lines from tampering would be too enormous, right?

There's my thought for the day.