You've listed some stakeholders, but there may be more possibilities (assuming sabotage). For example, it may benefit China to have their enemies fight, then o

You've listed some stakeholders, but there may be more possibilities (assuming sabotage). For example, it may benefit China to have their enemies fight, then offer assistance from the sidelines at a charge. Another option is that an energy supplier competitor did it, knowing that it would send the price of their energy through the roof. Even a few million dollars to have a private submarine sabotage the competitor would result in massive returns, and it would be difficult to trace. The point is, it's far too early to tell. Regarding your specific rational: > Ecoterrorists -- I discount this mostly on the basis that they would have claimed responsibility by now. I discount this on the basis of their capability and funding. > NATO sabotage -- this reduces strategic options for several EU countries It could be that a NATO Country (read: not EU) could want to see the collapse the EU. This would greatly benefit the US for example, as they would then be able to renegotiate all previous trade deals on favorable terms. > Russia sabotage -- this isn't terribly rational for Russia, but Russia has been unwilling to pursue rational options for a while. It wouldn't be, unless they have the ability to fix it easily. > Ukrainian sabotage -- pretty much the only actor for whom sabotage makes rational sense. Especially if it is a demonstration of Ukrainian cyberwarfare capabilities. Cyberwarefare didn't rip a hole in a pipe. Cyberwarefare also didn't create an explosion. I would be also interested to know if Ukraine has any operations submarines right now - because where would they refuel for example? > * Poor maintenance -- this strikes me as unlikely because NS1 is quite new infrastructure (2011), so it would have to be extremely substandard to have failed this quickly, and I believe most of the construction was done by European and not Russian firms, so I don't think that's at all plausible. But there may be some failure modes I'm not thinking of here. Over 10 years even the best infrastructure could deteriorate with abuse. Bridges can collapse if not well maintained within this timescale, and the ocean is a much worse environment.