Although the circumstances of the recent leaks in the North Stream pipelines in the Baltic Sea near Bornholm are still not totally clear, the temporal coincidence of multiple leaks suggest that some kind of sabotage might have happened. So far, there is nothing reliable known about the origin of the responsive party. It's interesting to discuss who would stand to benefit from such a sabotaging action.
Let's talk about the two players here who could do something like this
The United States
The US pretty clearly could do something like this fairly easily. The US leads the way in underwater technology and has military assets in the region.
The US sells natural gas, and destroying the pipelines would help accelerate the transition many European countries have been making towards adding liquefied natural gas(LNG) ship terminals.
“Around 25 new FSRUs, according to estimates from S&P Global Platts, are now expected to be installed across the EU in the coming years, with the first facilities set to be operational already by the end of 2022 facilitated by a combination of political and commercial will in accelerating work,” Illardo said.
The pipelines were seen as helping to keep European countries (Germany in particular, where the pipelines terminate) from involving themselves too much with Russia
By handing Putin such potential leverage over European energy security, it is argued, the 1,200 km pipeline leaves Free Europe at his mercy. If Putin wants a new Yalta, a new border settlement with Europe, then gas, and Europe’s dependence on Russian reserves, has become a means to achieve it. Nord Stream 2’s critics say it isn’t so much about creating additional capacity as it is about supplanting the main existing path for Russian gas to Europe, which runs through Ukraine.
Why not them?
US intelligence agencies warned many European allies this might happen. It would be strange to alert people to be on guard for an action you yourself are about to take.
LNG terminals mean anyone can sell you LNG. The US is not the only exporter of LNG.
It would damage NATO relations. The US is spending a lot on defending Europe and the US has gained more International goodwill
Perhaps even more substantial, however, was the shift seen across those who had been more critical than supportive of the U.S. role. The views of that role in Belgium, France, Germany, the Netherlands, Norway and Sweden changed significantly over the past year, and all now have slightly more positive than negative views of the U.S. influence on global democracy.
Denmark and Ireland went from negative to now being on the fence, while Spain, once directly in the middle, has now joined those more in support of Biden's democracy efforts than opposed. Spain, which was neutral last year, now sees the U.S. influence as slightly more constructive for democracy.
It would be strange for a president who is so environmentally conscious to intentionally release a ton of greenhouse gases
Russia started a war with Ukraine. It's been trying to use natural gas supplies to discourage NATO members from interfering and Germany in particular wasn't going to play that game, canceling the Nord Stream 2 pipeline
But following Mr Putin's order to send troops to the rebel-held Ukrainian regions of Donetsk and Luhansk, Germany announced it was suspending the process of giving [Nord Stream 2] between it and Russia an operating licence - effectively stopping the project until further notice.
The move is notable as Russia supplies Europe with about 40% of its gas, sourced from vast supplies in Russia's east.
Russia is sending a message that they can destroy undersea assets. In particular, Russia has extensively mapped undersea Internet fiber cables
In the worst scenario, an attack that cut submarine cables coming into the United States could significantly harm its access to the global internet. Ordinary users would probably experience massive losses in bandwidth, particularly for services such as Google and Facebook that host much of their data overseas. Since most government also relies on commercial internet infrastructure, the outages would also interrupt official communications. Any data hosted outside the U.S. would become nearly inaccessible to U.S. parties and vice versa. For people who depend on the internet in daily life, such an attack would be a national disaster.
The pipelines were shut down so Russia is not losing anything by destroying their own infrastructure
Why not them?
Russia made a lot of money from selling natural gas to Europe. While it was complicated by sanctions, exports were still up
Higher oil export volumes, coupled with rising gas prices, will boost Russia's earnings from energy exports to $337.5 billion this year, a 38% rise on 2021, according to an economy ministry document seen by Reuters.
The jump in revenues, if it materialises, will help shore up Russia's economy in the face of waves of Western sanctions.
There's no easy way to repair the pipeline either. This means Russia cannot sell Europe natural gas in the near-future in the event that Russia needs revenues in a post-war economy. Russia would have to contribute to any repairs.
I will try not to name single countries in this answer but rather only discuss capabilities and possible motives.
The question of capability clearly depends on the way the sabotaging act was done (if it was one and not just an unfortunate coincidence).
If some explosive devices have been attached, they could have been attached by a manned submarine, by an unmanned special-purpose submarine or by themselves (devices could maybe swim/dive on its own), even by humans (diving) would theoretically be possible in this depth of ~80m. It could have happened shortly before the leaks occurred or weeks/months before, maybe even already at building time of the pipelines or a short time later.
At the position (South of Sweden, East of Denmark, North of Germany, ...) there is lot of maritime traffic. It will be difficult to check all previous recorded traffic in the area and the traffic might come from many different origins.
How difficult it is to get a submarine there undetected and back is difficult to judge. Danish and Swedish underwater detection system should be state of the art with naval bases not far away. A smaller unmanned special-purpose submarine may be easier to navigate there and back again undetected.
Judging by the capabilities, every nation with submarines might likely be able to sabotage these pipelines, others rather not (so most of African countries and most landlocked countries probably not).
Also just sending a single submarine from Japan, China or India to the Baltic sea for such a critical mission sounds like it would rather be too risky to be detected on the way, so maybe further limit the capabilities to nations bordering the Baltic sea and France, Great Britain and the US.
As for the motive: There is no immediate direct economic damage, although the repair costs would surely have to be paid by the trading partners (Russia and Germany) and a repair would delay any possible delivery of resources. However these pipelines weren't used anyways currently and it's rather unlikely they would have been used this winter (the war between Russia and Ukraine is unlikely to end quickly and public support for the support of Ukraine in Germany is still relatively strong).
Therefore the damage is foremost of psychological nature. It increases uncertainty and further drives up prices of resources, increases inflation and the like. Every nation exporting resources would probably stand to profit from an increased uncertainty.
That leaves us with Norway, US, Canada, Russia but rather not with France, Great Britain or Sweden.
The last issue I want to point out is that if eventually evidence about the origin can be obtained, the sabotaging nation would lose a lot in reputation. Norway, the US and Canada would probably not want to risk that. Russia however would have much less to lose, because their international reputation is already quite ruined. But it still doesn't make much sense to blow up your own pipeline unless you think all is lost anyway (and top Russian may already be thinking that) or as a very, very indirect threat.
Finally it should not be excluded that not a country, but some kind of terrorist organization has done an act of sabotage there. Before 9/11 hardly anyone would have believed multiple airplanes can be kidnapped. Detonating explosives on an underwater pipeline sounds thinkable and doable although the symbolism of it would be difficult to explain (pipelines are bad?) and there might be much more "profitable" targets for terrorists available.
Finally the US has the most advanced military in the world, so maybe it would be easiest for them, but they definitely have other means to restrict usage of the pipelines, should the need arrive later, so the US would hardly have a good motive. The Russian submarine navy is also quite extensive and they have more bases close to the location of the leaking pipelines but also for them the potential immediate military gain seems to be rather low.
Summary: I think there are quite a number of countries with the capabilities (in principle) but none really which would clearly benefit. It rather feels like an terrorist act or the act of somebody who has nothing to lose and acts partly irrational than a well-thought through military strike (if it was one).
Well, in a nutshell, both Russia and at least some parties in the West have pointed fingers at each other, with press headlines like "Russian spy chief: West was behind sabotage of Nord Stream " or "‘Only Russia’ could be behind Nord Stream leaks, says former German intel chief".
Russian sources more specifically allude to the US as possibly having done this since the US was opposed to the construction of the pipes in the first place (at least of NS2 rather adamantly).
"US President Joe Biden must answer the question of whether the United States carried out its threat on September 25 and 26 when an emergency was reported at three lines of Nord Stream 1 and Nord Stream 2, which has been preliminarily recognized as ruptures, whereas he suggested those were blown up," the Russian diplomat said on her Telegram channel. On it, she posted a video in which the US leader delivers the speech in question.
"His statement of intent was backed up with a promise. One must be responsible for one’s words. Failure to understand what one says does not absolve anyone of responsibility. Europe must know the truth!" Zakharova emphasized.
Tucker Carlson has argued that same point at length, e.g. that "you'd have to be a suicidal moron to blow up your own energy pipelines [that] give you leverage over other countries", whereas "Joe Biden suggested on camera he might take out these pipelines." And he was rebroadcast on Russian TV.
And Putin has (a bit later) directly accused the "Anglo-Saxons" of having done this:
"The sanctions were not enough for the Anglo-Saxons: they moved onto sabotage," Putin said. "It is hard to believe but it is a fact that they organised the blasts on the Nord Stream international gas pipelines."
Some Western sources claim Russia may be interested in a false flag operation too since they have significantly reduced exports to Germany already, e.g. that "former German intel chief" (Gerhard Schindler) said:
The halt in gas supplies can now be justified simply by pointing to the defective pipelines, without having to advance alleged turbine problems or other unconvincing arguments for breaking supply contracts.
Additionally some Western newspapers (NYT) claim that
Both lines of Nord Stream 1 were damaged, whereas only one of Nord Stream 2’s lines was ruptured, which means that, at least theoretically, gas could flow through the second line.
Gazprom has confirmed this now, although they said they need to do a safety inspection on the (NS2) pipe that still had pressure/gas in it, so they are depressurizing it ATM.
In connection to that, in mid-July, amidst the NS 1 stoppages, Putin said that NS 2 was ready to deliver gas to Germany, despite the fact that it wasn't given German approval, while Germany insisted on Russia accepting the repaired turbine for NS 1.
As for capabilities, it's probably a bit premature to discuss since we don't know the exact methods used. According to one Western site, the pipes are only at around 50 meters depth around Bornholm, which would enable a fairly low-resourced attacker to carry this out.
The same Russian Foreign Ministry spokeswoman said the damage was in a zone where the US has "full control", meaning the EEZ of Denmark and Sweden. Western sources say that despite that proximity, Russian navy operates in this region as well, e.g. Denmark protested against an alleged Russian corvette intrusion in its territorial waters back in June in roughly the same area. (Russia denies that took place.)
As for how the US might have done it, from what I can tell Russian officials insofar have been vague (or if they've been more explicit, there's no coverage of them in English that I could find talking about means--although Naryshkin promised to reveal details at a later time.) On the other hand unofficial/suspected Russian sources/proxies like SouthFront and the US socialist website Monthly Review pointed to the USS Kearsarge, which was on tour in the Baltic Sea until recently. The Permanent Representative of Russia at the UNSC, Vassily Nebenzia, has now (Sep 30) mentioned Kearsarge's presence, and added
Most interestingly, the ship’s helicopter squad started patrolling the area around Bornholm as early as in August, and the flight line of those aircraft surprisingly coincided with the pipeline route. I emphasize that this is open data on geolocation of sea and air transport which is collected on the basis of the transponders’ signal. It means that the United States did not conceal its presence in the area and completed its maneuvers in an exhibitory and ostentatious way.
The latter source, MR, which published its piece one day before Putin explicitly charged the "Anglo-Saxons", also speculates that the Poles may have been involved, due to the proximity of their shore/bases, with or without US participation. They cite John Helmer for the latter variant, saying that "The explosions at Bornholm are the new Polish strike for war in Europe against Chancellor Olaf Scholz." (Poland doesn't have too good of a relationship with Germany right now, on a range of issues stemming from EU "rule of law" and their various forms of pushback, including war reparations.) Gazprom executives have complained in the past that the Polish navy was allegedly interfering with the pipeline laying process, although Poland denied that was the case.
There's also this rather interesting bit in Der Spiegel, which seems to be the only source I've seen that claims to know the exact contents of the CIA warning to Germany
The Germans were warned in summer by the CIA about a possible attack scenario on the Nord Stream pipelines. U.S. intelligence claimed to have intercepted Russian communications in which concerns were expressed about possible Ukrainian attacks on Western infrastructure. The Ukrainians allegedly tried to rent a boat in Sweden for this purpose. The CIA did not consider the scenario of a Ukrainian attack to be very credible, but the mere fact that the possibility of an attack on Western infrastructure was mentioned by the Russian side prompted the Americans to warn the Germans about the scenario.
It would have been quite a feat if Ukraine actually managed that. The same source mentions that the BND was also looking into Russian underwater robots that could have travelled all the way from Russia.
Not really an answer, but I've been actively learning Russian since ages ago and know a lot about Russia (see, e.g., this answer of mine), and I see some interesting indirect signs pointing to Russia. Let me start with this news:
Глава «Газпрома» Алексей Миллер сообщил, что не знает, как и зачем восстанавливать поврежденные нитки «Северного потока», если турбины для него находятся в нерабочем состоянии. (Source)
Here's my translation:
Head of Gazprom, Alexey Miller, has said that he doesn't know how and why to repair the damaged pipelines of Nord Stream, given that its turbines are non-operational.
For those who don't know, Germany actually begged Russia to accept a repaired turbine, but Russia used a formal pretext to refuse. This means that by the time of the incident, Russia had already demonstrated its lack of interest in keeping Nord Stream working.
And here's what Putin himself said about the incident (as translated into English on the official Kremlin website):
The attack on the Nord Streams has set an extremely dangerous precedent, which shows that any critical piece of transport, energy or communications infrastructure is under threat, regardless of its location, management or whether it lies on the seabed or on land. (Source)
Make no mistake - when the Russians feel wronged, they are very angry and vocal about it, and they retaliate in one or another way. The principle of reciprocity is a cornerstone of their foreign affairs. For Russians, getting wronged and doing nothing about it is tantamount to losing face. Putin is vengeful. And the reactions by Miller, Putin, and Russian news media, which are largely state-controlled, indicate that the Russians aren't really angry. There's no outrage. Putin's words can be seen as a veiled threat, "We did this and can do the same to Western pipelines."
I have very little knowledge about the investigation of the incident, but Russia's reaction looks very different from how I believe Russia would react to what it actually believed to be an external attack on its infrastructure.
One country with a motive that was not mentioned in previous answers is Ukraine. Already in 2014, the leader of Ukrainian nationalists threatened to destroy the pipeline for Russian gas supplies to Europe:
Yarosh threatened to destroy the pipeline for Russian gas supplies to Europe
The leader of the radical nationalist Ukrainian "Right Sector*" Dmitry Yarosh said that his organization, in the event of a possible conflict with Russia, would destroy the pipeline through which European countries receive Russian gas.
Yarosh made a threat to "destroy the pipe", because with its help "Russia earns money by distilling oil and gas," RIA Novosti reports.
Yarosh also demanded that the new leadership of Ukraine urgently form the headquarters of the Supreme High Command, declare a general mobilization of the population, take weapons deep into the country, and ensure the supply of weapons from NATO member countries.
As shown by the strike on the bridge in Crimea, Ukraine has the capability to make such attack.
Other answers have mentioned Russia, but the motives mentioned in those answers do not make much sense. The pipeline is more or less Russian property and Russia was the only country that wanted to complete this project. Destroying your own property does not make much sense. The only motive Russia could have is to use it as a false flag operation similar to the Gulf of Tonkin incident by the USA. Russia could then just blame Ukraine and use it as a pretext to start a war. Since there is already a conflict between Ukraine and Russia which can be called a war, this is not likely. Also, when talking about capability, the incident happened in NATO territory, so to get there for Russia would be problematic.
Even the NY Times is talking about a connection to Ukraine.
Note that I am not proposing this as the most likely explanation, just one possibility that has not been mentioned in other answers.
First, Russia has several times since the start of the war taken action to reduce supply via Nord Stream under the guise of "technical difficulties"; Europe claimed that these were a pretext and that the reduction in supply via Nord Stream was a deliberate action.
Russia might believe that they could benefit from cutting off supply via Nord Stream by being able to put pressure on Europe to bring an end to the Ukraine-Russia war sooner by agreeing to Russia's terms for ending the war (whatever those may be from time to time).
There are three major sets of routes through which gas is (or was) delivered to Europe from Russia: the two biggest are the Baltic Sea pipelines and through the Ukraine, with the set through Belarus being a much smaller player. (In fact, the Belarus route has regularly shut down, and even gone into reverse at times.
Removing the Baltic Sea route reverts the situation to something like that pre-Nord Stream 1, where the majority of Europe's gas from Russia (66% in the infographic below) transits Ukraine:
If Russia believes that Europe cannot significantly wean itself off of Russian gas, and most of that gas once again needs to go through Ukraine, a slowdown or stoppage of gas flow there (whether "naturally" due to the war or deliberately via Russian action that they blame on the war) creates pressure to end the war sooner rather than later. Given that it seems from the past few months that supporting Ukraine to better allow it to continue fighting is to the advantage of Ukraine and the disadvantage of Russia, Russia may see creating this situation as potentially advantageous to them.
Note the big "if" in the paragraph above; if Europe manages to wean themselves further off of Russian gas, shutting down Nord Stream may be of little help. Europe has been taking steps to do this but is not fully committed (Hungary being a particular example of lack of commitment).
There are political forces in Russia aiming to end the war more or less along Ukraine terms (by completely removing the army of Russian Federation out of the territory this country as it is most commonly recognized), see this answer. We do not know how much support do they have.
The Nord Stream I and II are capable of bringing huge money, especially if both are fully opened. This money can be used in a number of ways, from paying reparations for Ukraine till raising the living standards in Russia itself, to make sure the new government will be liked. EU may opt to support a new government by just paying for the gas they need to buy somewhere anyway.
Hence the destruction of the pipeline could have been ordered by the current government of Russia, even if it would be against the interests of this country. With the goal to make more sure there is no fast way back.
Gas pipelines have "mechanical pigs" that can move inside the pipe for inspection and maintenance. It may be possible that they can be used to deliver explosives anywhere along the pipeline. Only the country with full access to the pipeline could use this way to destroy it. Baltic sea is quite shallow and and ships/submarines attempting to deploy explosives would have been detected. Recently The Guardian also mentioned these maintenance robots,