Amazing Planet

We Still Don't Know What Causes The Glowing Milky Sea Effect

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Medieval maps showcased sea monsters that were regarded as legitimate hazards of oceanic voyaging, but their existence has been long-since debunked. But one mystery of the sea has yet to be fully explained: the glowing ocean water known as the milky sea effect. This eery phenomenon can be so large and luminous, it's visible from space.

Bioluminescent plankton. Illumination of glowing wave with long exposure, Krabi, Thailand.

A Sight Worthy Of Jules Verne

While the milky sea spectacle was mentioned in the 1870 novel 20,000 Leagues Under the Sea by Jules Verne, the first modern logs were recorded in 1995 by the S.S. Lima, a British merchant vessel in the Indian Ocean. The captain and his crew were surrounded by glowing water that "appeared to cover the entire sea area, from horizon to horizon." And knowing that it took the S.S. Lima a full six hours to cross from one edge of the glowing water to the other, it was quite the eerie scene.

Milky sea effect off the coast of Somalia in the Indian Ocean

The Mystery Continues

In 2005, Naval Research Laboratory scientist Steven Miller led the team that produced the first ever images of the glowing area. They found that it stretched an incredible 5,780 square miles—roughly equivalent to the land mass of Connecticut. So what causes the glow? "The circumstances under which milky seas form is almost entirely unknown," LiveScience quoted Miller as saying. "Even the source for the light emission is under debate."

The leading theory, however, is bacterial: specifically, it could be caused by a swarm of bioluminescent bacteria, which produce a continuous natural glow. Though it would have to be a very large bacterial colony, it is possible if there's enough food for them to eat—perhaps an algae bloom or some other mass of organic material. The fact that we still don't know just goes to show how little we know about the ocean. Maybe those sea monsters aren't so farfetched after all.

Watch And Learn: Our Favorite Content About Bioluminescence

The Milky Seas

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Key Facts In This Video

  1. The bioluminescent area of the Milky Seas can be seen from space. 00:01

  2. The bacteria they believe causes the Milky Seas is Vibrio Harveyi, and there are 40 billion trillions of them in the water. 01:26

  3. Jules Verne used real ship's logs to describe the Milky Seas in his book "20,000 Leagues Under the Sea". 02:14

The Glowing Bio Bay In Puerto Rico

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