From the article I linked to previously on laying the "Flag" cable in 1996.
Most of the fishing-related damage is caused by trawlers, which tow big sacklike nets behind them. Trawlers seem designed for the purpose of damaging submarine cables. Various types of hardware are attached to the nets. In some cases, these are otter boards, which act something like rudders to push the net's mouth open. When bottom fish such as halibut are the target, a massive bar is placed across the front of the net with heavy tickler chains dangling from it; these flail against the bottom, stirring up the fish so they will rise up into the maw of the net.
Also from the article
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Mere impact can be enough to wreck a cable, if it puts a leak in the insulation. Frequently, though, a net or anchor will snag a cable. If the ship is small and the cable is big, the cable may survive the encounter. There is a type of cable, used up until the advent of optical fiber, called 21-quad, which consists of 21 four-bundle pairs of cable and a coaxial line. It is 15 centimeters in diameter, and a single meter of it weighs 46 kilograms. If a passing ship should happen to catch such a cable with its anchor, it will follow a very simple procedure: abandon it and go buy a new anchor.
But modern cables are much smaller and lighter - a mere 0.85 kg per meter for the unarmored, deep-sea portions of the FLAG cable - and the ships most apt to snag them, trawlers, are getting bigger and more powerful. Now that fishermen have massacred most of the fish in shallower water, they are moving out deeper.
Formerly, cable was plowed into the bottom in water shallower than 1,000 meters, which kept it away from the trawlers. Because of recent changes in fishing practices, the figure has been boosted to 2,000 meters. But this means that the old cables are still vulnerable.When a trawler snags a cable, it will pull it up off the seafloor. How far it gets pulled depends on the weight of the cable, the amount of slack, and the size and horsepower of the ship. Even if the cable is not pulled all the way to the surface, it may get kinked - its minimum bending radius may be violated. If the trawler does succeed in hauling the cable all the way up out of the water, the only way out of the situation, or at least the simplest, is to cut the cable. Dave Handley once did a study of a cable that had been suddenly and mysteriously severed. Hauling up the cut end, he discovered that someone had sliced through it with a cutting torch.
There is also the obvious threat of sabotage by a hostile government, but, surprisingly, this almost never happens. When cypherpunk Doug Barnes was researching his Caribbean project, he spent some time looking into this, because it was exactly the kind of threat he was worried about in the case of a data haven. Somewhat to his own surprise and relief, he concluded that it simply wasn't going to happen. "Cutting a submarine cable," Barnes says,
"is like starting a nuclear war. It's easy to do, the results are devastating, and as soon as one country does it, all of the others will retaliate.------------------
I guess the nuclear war/retaliation theory of cable laying does not hold up as well against terrorists.
As for it's armor.
Shallow water is the most perilous part of a cable's route. Extra precautions must be taken in the transition from deep water to the beach, and these precautions get more extreme as the water gets more shallow. Between 1,000 and 3,000 meters, the cable has a single layer of armor wires (steel rods about as thick as a pencil) around it. In less than 1,000 meters of water, it has a second layer of armor around the first. In the final approach to the shoreline, this double-armored cable is contained within a massive shell of articulated cast-iron pipe, which in turn is buried under up to a meter of sand.
Also from the article on Flag, if anyone is interested on why and how they bury the cables.
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The injector is a huge steel cleaver, 7 meters high and 2 or 3 meters broad, rigged to the side of the barge so it can slide up and down and thus be jammed directly into the seabed. But instead of a cutting blade on its leading edge, it has a row of hardened-steel injector nozzles that spurt highly pressurized water, piped in from a huge pump buried in the Elbe's engine room. These nozzles fluidize the seabed and thus make it possible for the giant blade to penetrate it. Along the trailing edge of the blade runs a channel for the cable so that as the blade works its way forward, the cable is gently laid into the bottom of the slit. The barge carries a set of extensions that can be bolted onto the top of the injector so it can operate in water as deep as 40 meters,
burying the cable as deep as 9 meters beneath the seabed. This sufficed to lay the cable out for a distance of 10 kilometers from Tong Fuk. Later, another barge, the Chinann, will come to
continue work out to 100 meters deep and will bury both legs of the FLAG cable for another 60 kilometers out to get them through a dangerous anchorage zone.[/quote]
