though this doesn't show military cables, it reminds me a good book [1] about US eavesdrop operations against Soviet Union by literally surrounding their underwater cables with a special kind of "ring" that tapped electronic signals under it. (Fun fact was that USSR did not even encrypted those communications)
[1]: Blind Man's Bluff: The Untold Story of American Submarine Espionage by Sherry Sontag et al.
[1]: Blind Man's Bluff: The Untold Story of American Submarine Espionage by Sherry Sontag et al.