My first job was at an international carrier making the internet. Only a small bit of it but still, it was a great fun time. Line failures are common in this line of work. Someone opens up a cabinet a gives the wrong patch a firm tug or closes a door and something that wasn’t meant to bend ends up in a sharp turn. Sometimes a whole cable would fail, dug up by some nameless builder. Sometimes an undersea cable carrying millions of megabits per second of voice and internet between two continents would break. This is fortunately rare as there’s not much construction work under the sea. Most of time it’s a ship going by a coast dragging its anchor through the sea floor, sometimes a storm is so severe a cable gets tossed and turned until it breaks. When this happens there’s only one thing to do, call The Boat and wait two weeks or a month for it arrive, hoist up the ends and mend the cable.
But every once in a while something weird would happen. A cable would die and some hours later it would just come back online. When that happened the older more experienced cable maintenance guys would just shrug it off and say “it’s just the US army splicing in”. It was said half jokingly, half in awe. Splicing into an optical fiber is hard enough in dry land, cutting in and fusioning the light splitter is delicate work and doing it 100m under sea is nothing short of amazing. It involves cutting the cable in 2 and fusioning each individual fiber on both ends to the spliter piece that separates a bit of the light onto a second spy cable. You have to keep it dry throuhout the process and seal everything back up in the end. But the common wisdom was the americans could tap into undersea cables 15 years ago.
Or maybe it was only old linesmen tales and the americans never tapped anything.