Wrecks survive well in the Baltics because(at least a good part of) the bacteria that eats iron apparently does not survive well in saltless water.
Heres how well an actual WW I era submarine has survived the years:
https://www.youtube.com/watch?v=Xl5OA-MN-9sConsidering how small this new wreck is, only 20 meters in length, I'm willing to put my bet on USSR.
Yes, it crashed. In Baltic Sea. Same place where this "russian minisub" has been found, ya know.
There's a pretty dumb story behind how it got crashed, too. Got run over by a Sweden steamship while in submerged position.
It was never recovered, since there's no indication of that anywhere; ships are, by default, not recovered if possible. Especially considering the time period we're speaking about.
Yeh usually they just get declared as graves even if they are in shallow waters and recoverable. Subs getting run over by ships isnt all that silly... Theres at least one incident from the 90s where a Finnish cargo ship collided with a Swedish submarine. I couldnt find the pictures of the event right now but I've seen them in a blog, taken by the cargo ship crew. From what I recall Sweden even paid the repair bill.