Originally Posted by
etudiant
Surely the preferred option is to tap the cables, rather than to break them. A bomb will break any cable efficiently, no need for mini subs to do that.
Both the US as well as Russia have specialized bottom crawling subs whose most plausible routine use is to place and maintain cable tap installations.
The Royal Navy has form using X-Craft miniature submarines. They were required to bring back severed sections of the cables to prove the success of their missions:
Operations "Sabre" and "Foil"
These were a pair of operations to cut [Japanese] undersea telephone cables connecting Singapore, Saigon, Hong Kong and Tokyo during the Second World War. The intention was to oblige the Japanese to use radio and render themselves open to message interception.
Operation "Sabre" was carried out by "XE4", which was towed to within 40 miles of the Mekong Delta by the submarine HMS "Spearhead", where she looked for the two telephone cables by using a towed grapnel. She eventually snagged the first cable, and managed to haul it about 10 ft off the seabed. "XE4's" diver, Sub-Lieutenant K.M. Briggs, used the net/cable cutter to sever it. The second cable was soon found as well, and was severed by the second diver, Sub-Lieutenant A. Bergius. Two divers were carried due to the operating rule that a diver should not spend more than 20 minutes in depths over 33 ft and no more than 10 minutes over 40ft.
Operation "Foil" was carried out by "XE5", against the Hong Kong end of the cable, after being towed into position by the submarine HMS "Selene". Operating close inshore near to Lamma Island, working conditions were poor, "XE5's" divers having to work in thick mud under the constant threat of oxygen poisoning. Despite this, the operation was successful.