Milky Seas are a rare nocturnal phenomenon in which the ocean's surface emits a steady bright glow. They can cover thousands of square miles and, thanks to the colorful accounts of 19th-century mariners like Capt. Kingman, milky seas are a well-known part of maritime folklore. But because of their remote and elusive nature, they are extremely difficult to study and so remain more a part of that folklore than of science. Unlike bioluminescence that happens close to shore, where small organisms called dinoflagellates flash brilliantly when disturbed, luminous bacteria work in an entirely different way. Once their population gets large enough – about 100 million individual cells per milliliter of water – a sort of internal biological switch is flipped and they all start glowing steadily. Now imagine if a gargantuan number of bacteria, spread across a huge area of open ocean, all started glowing simultaneously. That makes a milky sea. Researchers think the purpose of this glow could be to attract fish that eat them. These bacteria thrive in the guts of fishes, so when their populations get too big for their main food supply, a fish's stomach makes a great second option.