The Arctic Ocean experiences yearly formation and then melting of massive amounts of ice on the sea surface.

The return of ice has closed both the Northern Sea Route, also known as the Northeast Passage, and the direct route through the Northwest Passage. This year was the first time since satellite measurements began that both passages were open at the same time, the ESA said Thursday in a release.

The Northern Sea Route extends from the Norwegian Sea through the Bering Sea to the Pacific Ocean. The Northwest Passage runs along the north coast of the North American continent.