For 500 years, explorers have looked for a passage above North America, from the Atlantic to the Pacific.

With a warming Arctic, they’ve finally got it. Which could have profound impacts on geopolitics.

In the 1500s, Spain and Portugal controlled the sea routes to their colonies in the Americas. Northern Europeans, looking for their own way to reach Asia, began searching for a Northwest Passage through frigid Arctic seas.

For centuries, voyagers endured winters frozen in ice, where entire crews died. Finally, in 1903, the Norwegian explorer Roald Amundsen, who was also first to the South Pole, picked his way through in a shallow-draft fishing boat, with a crew of just 6. But this was hardly a trade route.

In the last 10 years, summer ice in the Arctic has shrunk to its lowest volume since humans have measured it, finally opening a passage for cargo ships and tankers.

With this access, many nations have laid claim to the Arctic seas that touch their northern shores—and the rich mineral resources beneath them.

Russia is particularly interested in the oil there, thought to be 25 percent of the world’s remaining resource.

If ice continues melting, we may see an ice-free Arctic in 20 to 30 years. And growing international tensions as countries jockey for control.

It’s a new frontier, which could alter the balance of power in the Northern Hemisphere.