Explorers would take ships up Greenland’s west coast, then try to weave through Canada’s Arctic islands, before going down the Bering Strait between Alaska and Russia.The problem was that even in the summer the route was mostly blocked by impenetrable ice. Today, more than 170 years later, a warming Arctic means that the route is increasingly accessible for a few months each summer. And according to some estimates, Arctic ice is retreating to the extent that the Northwest Passage could become an economically viable shipping route.