Neal Stephenson always bears referring:
Mother Earth Motherboard:
http://www.wired.com/wired/archive/4.12/ffglass_pr.html
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.
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.
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