Waking at 10pm, a sailor looked out from the deck of the superyacht Ganesha to see that the ocean had turned 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,” they wrote.
For centuries, mariners have described navigating unearthly night-time waters, lit up by a mysterious glow, but such “milky seas” have long eluded scientific inquiry owing to their remote, transient and infrequent nature.
“I’d say there’s only a handful of people currently alive who have seen one. They’re just not very common – maybe up to one or two per year globally – and they’re not typically close to shore, so you have to be in the right place at the right time,” said Steven Miller, a professor of atmospheric science at Colorado State University in Fort Collins.
Milky seas are thought to be triggered by bioluminescent bacteria communicating with one another, possibly in response to changes in ocean currents driven by atmospheric conditions. Miller has been chasing them for decades, jealously listening to rare first-hand accounts and seeking scientific evidence to confirm their existence, as well as a means of seeing and studying the phenomenon himself.
“It’s a really huge and kind of mysterious response in our biosphere. We’d like to know how it works, and how that might change in a changing climate,” he said.
In the past decade, low-light imaging equipment fitted to newer environmental satellites has provided Miller with some tantalising leads. Now, witness testimony from sailors onboard the Ganesha has provided the first surface-based corroboration that these satellite images really are of milky seas – as well as the first real-world pictures of the phenomenon.
Between late July and early September 2019, US National Oceanic and Atmospheric Administration satellites captured what Miller thought might be a bioluminescent event south of Java, Indonesia, spanning more than 100,000 sq km (38,600 sq miles). In July 2021 he published images of the event – plus 11 other possible instances of milky seas – in Nature Scientific Reports. Media coverage of this research prompted Naomi McKinnon, a member of the Ganesha’s seven-person crew, to get in touch with Miller and describe the events they experienced on the night of on 2 August 2019.
The crew had been partway through a round-the-world trip when the Ganesha hit a patch of luminescent water between Lombok, Indonesia, and the Cocos (Keeling) Islands in the east Indian Ocean at about 9pm. The yacht entered these glowing waters suddenly, and the entire experience lasted until dawn.
One crew member told Miller that the colour and intensity of the glow was “akin to glow-in-the-dark stars or stickers”. The yacht’s captain said the glow appeared to originate about 10 metres below the water’s surface, rather than constituting a thin surface film as some scientists had imagined.
Lowering a bucket into the water revealed several pinpoints of steady light that darkened upon stirring – the opposite of what happens with “normal” bioluminescence, said Miller, whose findings are published in Proceedings of the National Academy of Sciences.
Pictures taken by the crew on a smartphone and a digital camera provided the first photographic evidence of a milky sea, he added. “To this point it’s all been word of mouth, dating back to the earliest times of the trade ships in the 18th century. They’ve all kind of described a similar thing, and the pictures are consistent with what they’ve described – it’s a kind of uniform, ethereal glow, almost a foggy appearance, very disorienting.”
This independent confirmation should make it easier to study milky seas in the future. “It means that we can now use [satellite images] with confidence to study milky seas from space, but also to direct research vessels equipped with the right kinds of equipment to sample the water and determine its composition,” Miller said.
“The 2019 Java milky sea appeared to last for at least 45 nights, which suggests these things are not just a shot-in-the-dark, one-night event, which would make it almost impossible to get out to one in time. We’ve found that when these larger ones get set up, they stick around for up to several weeks, if not a couple of months.”