A mystery that has puzzled sailors for centuries has been solved: This could be the origin of the sea’s phosphorescent glow

Typically, when sailing the oceans on a moonless night, while the heavens above maybe filled with stars, the water below is pitch black. However, every now and then sailors have witnessed events that made them think “everything had just been turned upside down, like the whole world was just awry.”

That’s because the water, for as far as the eye can see, begins to glow a green-white color that can be so bright that one can even read by its light. Known as ‘milky seas’, these events, which can even be seen from space, have been fascinating sailors for centuries and puzzling scientists.

New database will help scientists get closer to solving ‘milky seas’ mystery

The problem for scientists is that these illuminating phenomena are hard to predict. They generally occur in the Arabian Sea and the waters off Southeast Asia but they don’t happen in the same spot every year. Even where they occur frequently, there may be a multi-year gap between events.

However, scientists who want to research this phenomenon will now be able to use a new global database developed by Justin Hudson, a doctoral student in atmospheric science at Colorado State University, along with adviser CSU Professor Steve Miller. They analyzed 400 years of firsthand accounts of milky sea events, including ship logs and diaries, and satellite data.

They curated the database in the hope of finding correlations between conditions and locations so that researchers will be able to predict with better odds when and where they will occur. This will allow scientists to save precious research money and time as they seek to learn more about this maritime mystery.

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What causes milky seas? “believed” to be trillions of bacteria

Milky sea events can go on for months on end and cover an area of roughly 39,000 square miles. Hudson describes the spectacle like seeing the luminescence of “glow-in-the-dark stars that children put on their ceiling in their bedroom... and everywhere you look is just that even green.”

This is different from the more common blue glow that can be seen briefly in the ocean when the water is disturbed. Those bioluminescent displays are caused by a type of phytoplankton known as dinoflagellates and can often be seen along the shore as waves lap on the beach.

Speaking about his research on a CSU podcast he said that scientists “believe” that the milky sea phenomenon is caused by “very, very large numbers of bacteria.” Adding, “I think it’s in the trillions of bacteria per milliliter of water.”

This is based on a chance encounter by a research vessel with a milky sea event in 1985. “They found this bioluminescent bacterium called Vibrio harveyi around there, which lines up pretty well with what the glow of this milky sea looks like,” Hudson explained. “So, it’s why we strongly suspect this.”

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