Ytdyklly

Over 99% of the transcontinental internet traffic goes through a network of 280 sets of undersea cables.

How vulnerable are these to deliberate sabotage by the major powers?

I'd rather not focus much on why any nation would want to attempt such a thing or to what degree principles of MAD will apply, I'm just assuming that it's a war scenario and such intentional sabotage is desirable.

Such sabotage won't actually shut down the internet rather it would only slow it down. If the slow down is significant enough it could have serious impact on productivity, and it would cost more to send the same amount of data. Or certain kinds of internet traffic might get banned altogether by the governments in favour of maintaining higher bandwidth for more important traffic. Again this is another discussion that's out of the scope of this question, also it's not something that's easy to predict and will depend on the nature of the war/emergency.

I'm just asking how much would it cost to do it (sever say, 50% of the cables)? Is it possible at all? What technology would work best (I'm assuming some sort of robots but not 100% sure)?

Also take into account the fact that the sabotage operations will probably have to be geographically distributed and/or fast and/or discreet enough (with options for plausible deniability and blaming others nations for the sabotage) ... If you set up an entire city just to create the infrastructure required for the operation there's a good chance you could get bombed or nuked before the operation succeeds.

The power who wants to cut the cables is China. The U.S.A. has no reason whatsoever to cut the cables, because they control the core internet routing tables and they can obtain the same effect by tapping a few keys; Russia doesn't have enough naval vessels; India will never want to cut the cables because a lot of their income depends on providing high-quality low-price services over the Internet; Pakistan / Iran / other wannabes don't even begin to have the resources to do it.

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  "I'm just asking how much would it cost to do it (sever say, 50% of the cables)?"

  
  
  
  
  

  1. There is no point in cutting 50% of the cables. Either you cut all (or at leat almost all) the cables reaching the enemy or adversary, or the entire exercise is pointless. There is a lot of overcapacity, cutting half the cables will cause only very minor inconvenience.

  
  

  2. As an opening just before the shooting starts, it's not that hard for China to do. The cables themselves are not armored or guarded in any way, so all they have to do is send out a few hundred destroyers armed with long steel cables with hooks attached and drag them crosswise over the cable.

  
  
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     After the shooting starts, it's almost impossible to do. Any Chinese vessel trying to leave the protective arms of the mother country and reach open ocean will meet with a watery grave. The U.S.A. has satellites, and sonar networks, and submarines, and more naval vessels than everybody else combined.

     
     
     

     (Of course the war is China against the U.S.A. In a war with Russia there is no point the cut the cables -- the Russian Supreme Internet Censorship Authority (Roskomnadzor would actually thank you for such a service); the only other possible enemy of China is India, and the benevolent Chinese Communist Party knows very well that a war with India would be unprofitable.)

     
     
  
  
  
  


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  "Is it possible at all? What technology would work best (I'm assuming some sort of robots but not 100% sure)?"

  
  
  

  Surprinsingly primitive technology. Get a small-ish naval ship with a few thousand meters of steel cable aboard. Affix a bunch of hooks at the end of the cable. Throw the end of the cable with the hooks overboard. Drag the hooks across the cable. Bam, done.

  
  
  

  Don't try to be smart. Submarines don't work -- nobody has enough submarines to do this, not even the U.S.A. Robots are useless -- there is no point in using a very expensive robot when a hook at the end of a dumb cable will do the job.

  
  


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  "Also take into account the fact that the sabotage operations will probably have to be geographically distributed and/or fast and/or discreet enough (with options for plausible deniability and blaming others nations for the sabotage)"

  
  
  

  Well, duh. As soon as the Americans notice that somebody is cutting Internet cables wholesale the mighty U. S. Navy will become very very interested in finding out who exactly is playing with the livelihood of Amazon, Google, AT&T, etc. etc. The entire exercise is worthwhile only if all the cables can be cut more or less at the same time, and then only as a first act before an actually shooting war. As for plausible deniability, forget it. Ships are big are hard to hide.

  
  
  

  (P.S. That's why you need naval ships. The actions must be co-ordinated, and you really really don't want to tell civilians what you are up to.)

  
  

Less vulnerable than they were 10 years ago and it's getting better.

In 2008 I lost my phone. You wouldn't think anything of it, but it just happened that the same day someone dropped anchor on an undersea cable near Egypt. That anchor cut off the connection from the UK to the Indian call centre my mobile operator used so I couldn't contact them about it.

The powers that be are very aware of the vulnerability of the undersea cables. They used to primarily only guard them at the landing points, but they're also building up redundancy in the system as the connections become ever more critical to day to day life.

Some countries or regions will be easier to cut off from the rest than others. The islands (UK, NZ, Aus), isolated continents (Americas), those behind closed firewalls necessitating limited external connections (many Arab countries, China, soon Russia).

As mentioned in the comments, satellite connections do exist but they're far harder to access. Also weapons to solve that do exist, though only available to certain states.

In terms of the easiest way to do it, underwater ROV is effective but expensive. Dragging a large ship anchor through where the cable lies is also effective, but probably surprisingly expensive, large ships cost a lot to run.

Could a great power sabotage these cables? The answer is definitely yes. To do that, they would need submarines and/or ships that could drag a big anchor someplace on the ocean floor to cut the cable.

How hard would that be in a war scenario depends on the capabilities of who is going to war. If it is known that the cables are desirable to tap or cut, warships will be tasked with defending them. Then the effort required to cut them increases, because the force who wants to cut the cable will need to commit more resources to fight off the defenders and then cut the cables.

This is a problem that has been studied; the SOSUS warning system in the North Atlantic is essentially a series of cables with microphones attached to them, that were draped in several places in order to detect Soviet submarines. They would likely be targets for the Soviet Navy in the event of World War III. I do not know offhand how difficult it would be to cut those cables but I’m certain the Soviet Navy had a plan to do so and that the US Navy anticipated it. It might be a good case to research.

Search for phrases like "maritime cable cut" and top of the list just now was a BBC article from 2017 discussing just such a scenario. Sites like The Register regularly carry stories of accidental cable cuts - there are maintenance ships located around the world built specifically to locate and repair cables. I'm sure I've read other articles on The Register about how the cables are laid, repaired as well as how they have in the past been tapped.

In regards to cutting, as has been proven several times a ship dropping anchor or even trawling in the wrong place can do it easily. This shouldn't happen as the cable locations are known and charted. It doesn't take a lot to imagine somebody could send a fleet of "fishing boats" out to launch a coordinated cable cutting attack.

No great power or anything needed. Some storm, anchors and weights for nets of small fishing boats also do the trick, that even quite frequently as well as even thieves. One only careful can guess, but 30% are almost always more or less damaged.
This does not mean, there is no internet ... it just gonna be slower, especially during peak hours on international basis.
Internet uses some quite sophisticated protocols for it's traffic, 'scanning' and adjusting itself, similar to electricity or road traffic - following the ways of least resistance, managed by routers - 'Hey, have a long time not heard back, try another one'.
So in worse cases it even can be routed via power cables or electric fence, satellites and other media. Until routers have picked up new routes, delays/low speeds might happen, same as traffic jams occur and finally via side roads dissolve.
Can one really take a country take a country offline - ot really, as today's cables are just another medium for communication - today no bus, ok, take a car or even walk