Mareel

Milky seas, or mareel, is a condition on the ocean where large areas of seawater (up to 6,000 sq mi or 16,000 km2) are filled with bioluminescent bacteria or dinoflagellates, causing the ocean to uniformly glow an eerie blue at night. The condition has been present in mariner's tales for centuries – notably appearing in chapter 23 of Jules Verne's Twenty Thousand Leagues Under the Sea – but until recently it has not been rigorously documented.
The phenomenon is known as mareel in Shetland. This term is derived from the Norn word *mareld, which is itself derived from the Old Norse word mǫrueldr, which is a compound of marr (mere, sea) and eldr (fire). [a]