We visited Gdansk in January 2010 when the daytime temperature dropped as low as minus 28 degrees Celsius. The harbour was completely frozen over with a thick layer of ice and snow. People were to be seen walking across the frozen harbour, seemingly confident that the ice wasn't going to break under their weight.
I had visited Gdansk a few years previously, in August, and taken a boat trip from the harbour. It was difficult to believe that this was the same place!