Transatlantic Cables versus Nuclear Submarines

Something struck me whilst watching The Spy Who Loved Me a while back. When companies lay transatlantic cables, or perform undersea work anywhere, what mechanism prevents them from interfering with submarine military operations? Given that nuclear submarines are supposed to keep quiet and not give away their presence or future position.

Presumably, if a British company (for example) lays a cable in the Persian Gulf, they have to notify a central authority, and the Royal Navy would keep tabs on the work, but what prevents (for example) an Iranian submarine from blundering into the work area accidentally?

Presumably the odds of a submarine actually hitting a cable as it’s being laid are tiny, but an accidental near miss might cause an international incident. I imagine the submarines themselves are able to detect and avoid undersea construction work - which would be very noisy - but if that was the only safeguard, I can imagine a hostile power easily exploiting it (by trailing one of their own submarines behind a cable layer, for example).

I can think of several ways of avoiding this problem, but I’m genuinely curious as to how its done.

Ocean’s big. Cable laying ships are noisy. Subs stay out of the way.

And if you want to fake an international incident, plenty of ways to do so without putting a submarine crew at risk.

Off-topic, but related: the book “Blind Man’s Bluff”, all about spying subs versus underwater cables during the Cold War, is outstanding.

Also off-topic, but related: Mother Earth Mother Board, which explains some of the history and technology behind the existing network of undersea cables. If you read that, you’ll see that for cable-laying crews, bumping into military submarines - which are both interested and able to stay the hell out of the way - is pretty much the least of their concerns.

With regard to the Persian Gulf, on a calm day a lookout could just look down and see the sub. It’s only 90 metres deep at most.

Also, the work mostly isn’t done underwater.
The cables or amps are located with hooks & pulled up onto the deck of a ship, where the necessary repairs are done. There’s enough slack in the cqbles to do that, and the work is much easier to do out of the water.

That seems like it would make the situation worse. A vessel that’s actually laying cable would be making a lot of noise; engines, propellers, and such. If it’s doing maintenance and hauls the cable on deck, then it would have to keep still. That would probably be quieter than being underway, and there would be two lengths of cable running down to the seabed. I know it’s a one-in-a-million (and then some) chance of the sub running into the cable, but do they have any means of detecting it under those circumstances? I doubt they’d give their position away with active sonar. Are there magnetic or electrical emissions that could be detected?

Didn’t stop a french SSBN bumping into a UK one in 2009.



Most submarine cable operations require permits from the appropriate national authority. That’s why it can take some time for repairs to be commenced: the local authority hasn’t issued a permit yet.

So, the authorities will advertise that work is taking place in a certain position, to avoid having shipping running into a stationary cable ship. If the cable is located in the high seas, the repair ship still has a high interest in letting everyone who may be in the area know where the ship is.

I work for a telecommunications company and am always getting emails regarding cable failures and repairs from the cable operators such as SMW3, China-US, and so on.

Any vessel that would be at risk of running into a cable would also be at risk of running into reefs, icebergs, other ships, and the ocean floor. They have to have some way or another of detecting their surroundings, and if they fail to use them, that’s just the military version of Darwin in action.

True, but reefs are generally static, and icebergs and ship hulls only descend so far into the depths. That’s general subsea navigation of a sort that submariners occasionally mess up, with career-terminating consequence. I’ve always assumed that once under way nuclear submarines rely on charts, passive detection, and pre-arranged “avoid this spot” notifications in order to avoid hazards rather than beaming out active sonar, which would give away their position. Like a man navigating a dark room by remembering where the furniture is and counting his steps, instead of using a torch - and hoping that no one has moved the couch.

I can imagine a permit notification system being used offensively. If there are two main undersea routes past a certain obstacle, a hostile power could arrange for work to be undertaken in one of the routes; or they could set up a notification at very short notice, and then monitor undersea communications, looking for any unscheduled messages. Might not give them a submarine’s position, but every bit of data on the enemy’s communications would help.

How do they determine where the break is? There is a method of putting a bridge on land lines, but I don’t recall how that works. It probably relies on access to other lengths of the cable.
Which brings to mind the question, are undersea cables copper or fiber optic?

Notices to Mariners.

The old ones were copper; new ones (everything put in within the last decade or so) is fiber optic.

Most of the old copper ones are no longer used – there is so much capacity on the fiber ones that it wasn’t worth keeping the copper ones in operation. So now they just lsay there, on the bottom of oceans. (I wonder when they will be pulled up again, just to recycle the copper in them?)

I doubt that would be worth the expense. OTOH, they are very long. :slight_smile:

With fiber, do they have to pull up and examine the whole cable? That would fellate with great alacrity.

I believe the fiber ones have amplifier/regeneration devices built into them at various points. So I would expect that with current technology, they can query to these devices, and see which ones answer. That would allow them to isolate the section of cable that is damaged.

(With the old copper ones, they had a way of measuring the current going into the cable, and estimating from that how far it was until the break. Seems like magic to me, but they did it.)


Yes, the copper would be shorted at the break, so a bridge would measure resistance, a function of length. Also cool. :slight_smile:

I’ve read it. Excellent book, highly recommended.

This is a neat map of undersea cables.

What you’re referring to I believe is the Wheatstone bridge.

Yes, thanks.