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That's impossible because it is an industrial area.

It might be toxic lake?

No, I don't think so. It's the pure chalk that's make the water that kind of blue. It isn't real blue, it's just because of the light

OK, cool. Maybe it's even medicinal?! 😅

No, I don't think so. It's the same phenomena as
blue lake.jpg
here: The Blue Lake of Germany

WoooW! It's so beautiful! I've seen these beautiful blues and greens only in seas. I didn't know lakes of such colors. Thanks for showing us.