Polynyas are areas of ice-free water in the Arctic ice pack that stay clear for up to 9 or 10 months of the year. These openings are caused by ocean currents, wind patterns, tides, or upwellings of warmer water.

The areas of open water vary in size, and some can be huge - spreading over an area of up to 50,000 sq. km / 20,000 sq. miles.

Some polynyas occur at the same time and place each year (so regularly that they're even mapped and named), and the open waters provide a predictable overwintering ground for marine mammals such as walruses, narwhals and belugas that do not migrate south.

In spring, the thin or absent ice cover allows light to penetrate the ocean's surface layer as soon as the winter night ends, and this triggers the early blooming of microalgae that are at the base of the marine food chain.

It is through openings such as this that submarines like the U.S.S. Skate sometimes surface. Subs are known to pop up through the Arctic ice from time to time, and openings that are already mapped just make things easier.