You think that Sweden and Denmark surveil every single dinghy and fishing boat and yacht and ferry? That if a tugboat stops for 3 or 4 or 8 hours along the 2400km stretches of the Nord Stream pipelines, the coast guards will swoop in with helicopters and snipers. Yeah, no. No, that would not have happened.
No, but they definitely have the possibility after the event to replay recordings from their very nice naval radar systems to see if there was a surface vessel doing something in those locations.
Also, I understand the depth of the pipeline is 80-100 meters so you need divers experienced with hypoxic trimix (or an ROV) and you need to be able to carry a 50-100 kg payload in addition to all the diving kit. Just to qualify for the training courses to learn this type of diving you need to have logged many hundred dives with gradually increasing equipment complexity, it takes years and years of training.
I don't buy for a second that this is anything else than a nation-state actor with advanced equipment. The community of people in the world whou could dive to these depths and perform any kind of useful work is tiny and close-knit.
I'm one training short of hypoxic trimix (after a regular trimix course). I think you seriously over-estimate the complexity: open-water course is 3 days, advanced open-water is 2 days, nitrox is ... half a day?, basic tech diving (advanced nitrox + decompression procedures) is 7 days (50-100 dives minimum iirc), trimix is another 7 to 10 days.
Loads of people I know did all that without any difficulty and without leaving their day job. They weren't in the navy either.
The most advanced thing that I see here is how do you trigger the explosion underwater, plus of course getting the explosives somewhere.
Yeah, maybe I am over-estimating, I only have advanced open-water. But I thought you had to have logged like 50 dives at X depth between each step up the training ladder for trimix, so you need a minimum of like 200 dives total to reach hypoxic trimix? In my book that is multiple years of practice and not everyone will cut it.
So an organization either needs to make preparations for this many years in advance, or try to recruit someone who both has some form of extremist beliefs and a hobby that requires very specialized training. Neither of those things are very simple, but the first one is doable, yes.
