I've seen an internet cable over a bridge on TV once (New York?). By this, I have an idea for safe internet cabling (intrusion secure by illegal access), in the hard sense, by physical cables.
The physical cables are simply shielded, whatever they contain, fiber or something else, by an outer layer of electricity carrying material that is monitored for abnormal fluctuations indicating breach of integrity and attempt of getting access to the (data) cables!
This is merely an idea. I don't know if you would rather use magnetic or electrical detection.
You can investigate yourself. Good luck!
Endnote: This idea is likely to carry strict engineering requirements. All from the right detection strength to insulation that produce the sufficient quality for this idea to happen!
Perhaps the shielding can carry 10 000 Volt and merely signalling Amperage... Your job to find out... (or mine if I get there)! Cheers!
Perhaps a possible breach could cause "earthquake effect" and thus you would know where the breach would be in the system.
...On this line! Other alternatives are direct surveillance monitoring of the cable areas or some patrolling of kind, by security company or all 3 combined! Whatever the degree of security you want! (2 comments more made to Facebook.)
Note: First posted to Facebook some 10 minutes ago (from 9/07/2011 10:28:00 PM).
Note2: two more comments first made to Facebook with datestamp on the comment below.