Blue ice

occurs when snow falls on a glacier, is compressed, and becomes part of a glacier that winds its way toward a body of water (river, lake, ocean, etc.). During its travels, all of the air bubbles that are trapped in the ice are squeezed out, and the size of the ice crystals increases, making it clear.

The blue color is often wrongly attributed to Rayleigh scattering. Actually, the ice is blue for the same reason water is blue. Namely, it is a result of an overtone of a OH molecular stretch in the water which absorbs light at the red end of the visible spectrum.^[1]

Blue ice covering Lake Fryxell in the Transantarctic Mountains, Antarctica

Runways

Blue ice is exposed in areas of the Antarctic where there is no net addition or subtraction of snow. That is, any snow that falls in that area is counteracted by sublimation or other losses. These areas have been used as runways due to their hard ice surface which is suitable for aircraft fitted with wheels rather than skis.


http://en.wikipedia.org/wiki/Blue_ice_%28glacial%29