Comment by londons_explore

The original version of this post did have a repair plan.

Basically, every few kilometres you turn off the surface hardening of the cable for a yard or two. That spot won't propagate cracks - which means that if someone destroys part of the cable, the rest will be fine.

Those spots of cable have no tensile strength, so you wrap just those spots in a post tensioned steel sheath.

Then, you also make a few spare kilometers of cable that you lay in the ocean floor. When an incident happens, tow a new cable into position and connect it up. Underwater glass forming is a silly idea - but you can simply crack away the glass at the ends, reconnect the aluminium, then encase the whole thing in a couple of yards of epoxy.

The above plan I considered probably was of similar cost to simply laying a new cable across the entire ocean ahead of time in preparation though.