For centuries, sailors have been reporting an odd phenomenon: the entire ocean glowing in marvelous white light extending as far as the eye can see, earning it the name of the ‘milky sea’ effect. These very rare sightings could be dismissed as the mad ramblings of intoxicated seamen had they not been confirmed by entire crews.

These milky seas, sometimes called mareel, can extend over tens of thousands of square kilometers (as large as some countries) and are bright enough to be seen by modern satellites. They’ve been described as sailing through a snowfield or cloud tops. Now, scientists have poured through countless satellite images hunting for milky seas and have for the first time gained the opportunity to directly study these fleeting phenomena.

In the process, scientists have confirmed long-held assumptions about milky seas that they’re caused by bioluminescent bacteria. Billions of trillions of them, actually. And that’s just the beginning.

Turning the lights on the dark sea

“The sea was luminous in specks and in the wake of the vessel, of a uniform slightly milky colour. When the water was put into a bottle, it gave out sparks…”

The above quote is actually the very first entry in Charles Darwin’s zoological notebook written during his iconic trip aboard the Beagle, just off the coast of Tenerife, on January 6, 1832. What the British naturalist saw was a prime example of bioluminescence, which refers to light produced by a chemical reaction within a living organism.

Bioluminescence is so common and useful to living things that scientists believe it evolved independently at least 40 times both on land and in the sea.

More than 75% of deep-sea creatures are estimated to produce their own light. The anglerfish, for example, uses bioluminescent lures which resemble fishing rods, to attract prey. Closer to the sea surface, bioluminescence is generated by plankton called Noctiluca scintillans, commonly known as “sea sparkle”. 

However, milky seas are a special treat since they’re incredibly rare. On average, they’re only sighted twice a year, and mostly in waters around the northwest Indian Ocean and right off the coast of Indonesia. They also cover much larger distances than plankton glows.

Satellites shine light

Since they appear seldomly, milky seas have always proven to be a bit of a mystery but modern satellite readings may change that.

Using NOAA weather satellites, researchers have trained computers to identify milky seas in real-time, opening up the possibility of studying these elusive phenomena before they disintegrate.

“Now we have a way of proactively identifying these candidate areas of milky seas,” said Steve Miller, a senior research scientist at Colorado State University and lead author of the new study, which was published in Scientific Reports. “If we do have assets in the area, the assets could be forward-deployed in a SWAT-team-like response.”

Miller explains that milky seas are produced by the luminous bacteria Vibrio harveyi. These bacteria stay dormant until their numbers swell past a critical threshold –100 million cells per milliliter of water. Once this threshold is crossed, it’s like a biological switch is flicked and the bacteria turn the ocean into a milky sea.

Scientists think the bacteria do this to purposefully attract the attention of fish that eat them. That sounds very odd and counterintuitive at first glance, but it’s actually a pretty clever ploy. The bacteria swell to huge numbers by feasting on massive algal blooms, but this food soon runs out. Once inside the fish’s stomach, the bacteria thrive in the guts just like many other bacteria live inside our bodies.

Miller first started using satellite imagery to explore the milky sea effect in 2004. However, the technology back then was subpar, so the observations have proven unclear and impossible to work with. That changed with the Day/Night Band instrument that went live in 2011 aboard NOAA’s Suomi National Polar-orbiting Partnership (NPP) and the Joint Polar Satellite System (JPSS) satellites. This Day/Night Band (DNB) instrument can break down light into gradients, allowing satellites to see through the lights of cities and wildfires.

The researchers established which were the most common spots where milky seas have been sighted over the last 200 years, then concentrated on these regions. This hunt proved fruitful and Miller and colleagues identified over a dozen milky sea events between 2012 and 2021, the largest of which occurred south of Java in 2009.

NOAA satellite imagery confirmed this particular milky sea grew to monstrous proportions, extending over more than 100,000 square kilometers, roughly the size of Kentucky. The number of bacteria participating in this event is simply inconceivable. Perhaps 100 billion trillion cells were involved, which is about as many as there are stars in the observable universe.

These observations helped to unravel many previously mysterious aspects of milky sea formation. For instance, water temperature and chlorophyll are very important.

Other questions remain unanswered, though. These include the thickness of the milky broth and uncertainties over whether algal blooms are indeed the bacteria’s primary food source. Scientists would have to find themselves in the middle of such a milky sea to be better equipped to answer these questions.

“Perhaps the most practical revelation is how long a milky sea can last. While some last only a few days, the one near Java carried on for over a month. That means that there is a chance to deploy research craft to these remote events while they are happening. That would allow scientists to measure them in ways that reveal their full composition, how they form, why they’re so rare and what their ecological significance is in nature,” Miller wrote in an article.