A block of sea ice floats in the wake of the Finnish icebreaker MSV Nordica as it sails through the Victoria Strait while traversing the Arctic's Northwest Passage, Friday, July 21, 2017. Sea ice forms when the top layer of water reaches freezing point, usually around the start of October. As temperatures continue to fall this first-year ice grows downward until it is several feet thick. If the ice survives the following summer melt it becomes second-year ice. Another cycle and it becomes multi-year ice, which is the toughest kind. (AP Photo/David Goldman)
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