Many people think that once a sea freezes, it is one complete sheet. In reality, there are several open areas that never freeze. One such area is a polynya (common US spelling) or polynia (common UK spelling). It is a loanword from Russian: полынья (polynya), which refers to a natural ice hole, and was adopted in the 19th century by polar explorers to describe navigable portions of the sea.

Polynyas are large, persistent regions of open water and thin ice that occur within much thicker pack ice, at locations where climatologically, thick pack ice would be expected.