On some moonless nights, enormous patches of the Northwest Indian Ocean and seas around Indonesia begin to glow. This event has been witnessed by hundreds of sailors, but only one research vessel has ever, by pure chance, come across this bioluminescent phenomenon, known as milky seas. Thanks to that vessel, samples showed that the source of the light was a bacteria called V. harveyi, which had colonized a microalgae called Phaocystis. But that was back in 1988, and researchers have yet to be in the right place and the right time to catch one of these events again.

Both the bacteria and algae are common to those waters, so it’s not clear what triggers these rare events. To help understand why milky seas form, researchers have gotten much better at spotting these swaths of bioluminescence from the skies. With the help of satellites, Stephen Miller, a professor of atmospheric science, has been collecting both images and eyewitness accounts of milky seas for nearly 20 years. Thanks to improvements in the imaging capabilities over the past decades, Miller published a compilation last year of probable milky seas in the time frame of 2012 to 2021, including one occurrence south of Java, Indonesia, in summer 2019. //

Although milky seas can be massive—greater than 100,000 square kilometers in the case of the 2019 sighting—the intensity of this bioluminescence is still relatively faint. By comparison, the better-known sea sparkle from marine plankton (dinoflagellates) is 10 times stronger—and even that can be hard to spot. //

“When waking up at 2200 the sea was white. There is no moon, the sea is apparently full of ? plankton ? but the bow wave is black! It gives the impression of sailing on snow!” the Ganesha crew wrote in their logs.

This glow was continuous as far as they could see, and they also compared the effect to glow-in-the-dark stickers. When they collected some of the water in a bucket they found that the light extinguished when stirred, which is the opposite of dinoflagellate behavior.

“I was surprised by their description of the pinpoints of light that vanished upon stirring and the sense that the glow was coming from depth,” writes Miller. “The disappearing glow may be due to bacterial communities being broken up that made their individual glow less apparent than when concentrated on a particle, or a change in the water that shut off that glow… not sure!” //

“I would also like to point out that while we look in collective awe at the incredible photos from James Webb Space Telescope coming from the edges of our universe, there still somehow remain these fascinating mysteries down below, waiting to be discovered, appreciated, and learned about,” adds Miller.