I was mostly curious to know what those objects were (practice demo charges, perhaps?) and why the names on them (the last USS
Skipjack was decommissioned in 1990; the only USS
Flounder was removed from service in 1947).
One suggestion I've seen based on the footage was some kind of linear shaped charge fitted across the pipeline, which might explain the clean cuts and that there appears to be a rectangular strip peeled out to the side. I don't agree that it was necessarily done from a sub; as stated, a nuclear boat going in is unlikely and diesel-electric coastal boats don't have a lot of space for cargo and passengers (also, except for specially-configured boats hatch/escape trunk size is a constraint). Additionally, the sabotage was done in two locations; Russia and Poland each have only one sub based in the Baltic (both 1986-vintage Project 877 Paltus/Kilo boats, amusingly enough). The only other navies that operate subs based in the Baltic are the Germans and the Swedes; the Norwegian sub fleet is based at Bergen on the North Sea.
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More likely suspects could be those two surface ships observed in the area beforehand that were not broadcasting their positions on AIS; one hint might be the estimated size from satellite imagery of 95 to 130 meters. That's larger than either the Polish
Kormoran-class minehunters or
Piast-class salvage ships, which would be the most likely Polish candidates to support underwater demolition operations. It might be the right size bracket for their largest ships, the
Lublin-class minelayer landing ships (odd combo) and
Perry-class frigates, but those aren't exactly great diving support platforms.