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We can consider the Millennium Falcon in the Baltic.
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It really doesn't look volcanic to me. It doesn't look particularly like a building to me either, but I could see it as some sort of huge platform, a kind of elevated park or something. I don't think it's a crashed spaceship or anything of that ilk, but I don't think it's happenstance either.
It's at least from before the last ice age, maybe two of them ago. Rocks dropped on top of it make this clear. Some of them are from very far away. It's shaped a heck of a lot like the Millennium Falcon, and seems cement-like, but it's held up off the seabed on a pedestal, like a huge mushroom.
So maybe it is a petrified docked spaceship... the remains of Star Fleet Command... but....
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There's a theory that it may be the gun turret from a battleship in WWII. Got blown off the ship and sank with the guns pointing down. This is a good theory... if the gun turrets were in fact that big.
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