Beyond the Arctic belt of ice lies a large, navigable open sea surrounding the North Pole.

The Open Polar Sea became a serious nineteenth-century geographical hypothesis and influenced actual exploration.

Supporters argued that warm currents from the Atlantic might keep the central Arctic ice-free. They pointed to migrating birds flying north, summer’s continuous sunlight, the assumed inability of icebergs to form far from land, and occasional stretches of open water beyond heavy pack ice.

Those observations were real; the inference was wrong. A polynya, a local area of open water surrounded by sea ice, could look tantalizingly like the edge of a larger ice-free ocean. Elisha Kent Kane’s 1850s expedition returned with reports of open water to the north, strengthening the theory instead of killing it.

The hypothesis became tied to scientific prestige, commerce, imperial competition, and the search for navigable polar routes. German geographer August Petermann gave it additional scientific authority in the 1860s.

Exploration eventually supplied the missing scale. Expeditions continued to encounter enormous moving fields of sea ice across the central Arctic. The ill-fated Jeannette expedition, which departed in 1879 expecting open water, instead became trapped and drifted in pack ice.

Modern climate warming is now making the Arctic increasingly seasonally ice-free, but that future phenomenon should not be confused with the historical claim that a naturally navigable open sea already existed beyond a ring of ice.