It certainly has been the case in the past (when undersea fiber operators were much less careful) that cables have been tapped without the owner getting wise.

It certainly has been the case in the past (when undersea fiber operators were much less careful) that cables have been tapped without the owner getting wise. IIUC the method used in the past was to bring the cable inside a submarine which has a specialized fiber cleaving and joining machine. Some amount of full transmission loss already occurs, so to the operator is just looks like blip. Here's a description of an early operation (which I think was actually on copper cables): https://en.wikipedia.org/wiki/Operation_Ivy_Bells https://en.wikipedia.org/wiki/Operation_Ivy_Bells When I worked at Google, Snowden and others showed that it was likely the US NSA was spying on Google fiber outside of US, I believe the speculation was that they tapped lines around UK, possibly underwater. There's nothing quite like seeing a packet trace containing an RPC between a frontend and backend and being able to recognize the communicating services, collected by a third party. Google greatly sped up its RPC encryption project after that revelation.