The US famously spied on Russian underseas cables in Operation Ivy Bells https://en.wikipedia.org/wiki/Operation_Ivy_Bells https://en.wikipedia.org/wiki/Operati

The US famously spied on Russian underseas cables in Operation Ivy Bells https://en.wikipedia.org/wiki/Operation_Ivy_Bells https://en.wikipedia.org/wiki/Operation_Ivy_Bells during the cold war. A spy sold the secret to the Russians and they recovered the recorders. Of course they used submarines, so there was no plot on normal publicly-accessible marine maps.. The Russians have several subs with mini-subs too and are building more; there is a good list of links at the bottom of the article!