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I read something about st Petersburg originally being build on a swamp?

More or less correct. It's not like Holland, whose territory is largely below sea level, but there are sufficient parallels... and it has not been washed away ( so far :))) but really suffered a lot from floods. During its entire history there were more than 300 floods! and several of them were Great Floods. By the end of XX century, eventually it was protected by a dam, now floods do not threaten it.
PS
I currently work on a book about the Great Flood of 1924 (I do the layout and design of the album). I will definitely make some post about it, I have such a mass of hot photos at my disposal that my hands are just shaking from the desire to share it all :)))