Slavic etymological goodie:
The name Plön, with it's lovely castle, so central in the history of the Holstein-Gottorps, allegedly comes from Polabian plun'a, ice-free water, referring to the lake. No trace of a such word either in Polish or Russian. (In Russian it's prorub', in German it's Wake, in Swedish vak, Norwegian råk) But then - in Ukrainian ice-free water or hole in the ice is opolonka. This gotta be the word, having gone through pleophony!