Milky seas (Somali: Kaluunka iftiima; English: Milky seas), also called mareel, is a luminous phenomenon in the ocean in which large areas of seawater (up to ) appear to glow translucently (in varying shades of blue). Such occurrences glow brightly enough at night to be visible from satellites orbiting Earth.
Mariners and other seafarers have reported that the ocean often emits a visible glow which extends for miles at night. In 2005, scientists announced that for the first time, they had obtained photographic evidence of this glow. It is most likely caused by bioluminescence.