Science & Nature - Water sky: how clouds guide polar explorers to open sea

In polar regions, water sky is an optical phenomenon that reveals open water hidden beyond the horizon. When sunlight strikes dark ocean water, some light bounces upward onto the undersides of low-lying clouds. This reflection produces noticeable dark patches under the cloud cover. Polar explorers, including Fridtjof Nansen in the Arctic and Douglas Mawson in Antarctica, relied on these telltale dark spots to find navigable lanes through sea ice. Measurable differences in the polarization of light make the phenomenon visually distinct.

Source: Wikipedia