As sea ice in the warming Arctic retreats more and more during summer, the fabled Northwest Passage is becoming a greater temptation. The route—actually a series of straits across northern Canada—would cut 3,000 miles off the voyage from New York City to Shanghai via the Panama Canal. But in practice, lingering ice is so unpredictable that a crossing remains risky and expensive. Arctic scientists think it will be many years before ships can make the passage regularly.

Credit: Katie Peek; Courtesy of NASA Worldview (satellite images); Sources: U.S. National snow and ice data center (annual ice extents); Canadian Ice Service (Viscount Melville Sound); FEDNAV (Nunavik); Crystal Cruises (Crystal Serenity)