For centuries, sailors have reported eerie encounters with vast stretches of glowing oceans—what they described as “milky seas”—lighting up the water with a ghostly white or green sheen. Now, thanks to a landmark study published in Earth and Space Science by researchers at Colorado State University, science may finally be closing in on the source of this elusive natural phenomenon.
A Rare Glow with Bacterial Origins
Milky seas are an exceptionally rare type of bioluminescence, likely caused by colonies of Vibrio harveyi, a type of glowing marine bacteria. Unlike the flashing sparkles caused by plankton, milky seas emit a steady, even light that can spread across thousands of square miles and remain visible for days or even months.
“It looked as if we were sailing over a boundless plain of snow,” wrote one captain in 1849—a sensation echoed by others across centuries of maritime records. Witnesses describe the ocean glowing like “green neon lights” or “molten lead,” creating a surreal sense of calm. That stillness may not be imagined: researchers found that certain algae involved in these events emit mucus that calms the sea’s surface, possibly enhancing the illusion.
Building a Database from 400 Years of Clues
The research team—led by Justin Hudson, a PhD candidate, and Steven Miller, an atmospheric scientist at CSU—spent months combing through eyewitness accounts, ship logs, newspapers, and satellite data to compile the world’s first large-scale database of milky sea sightings. In total, they found fewer than 400 reliable cases across four centuries.
“People without scientific training capture the information completely differently,” said plankton ecologist Abigail McQuatters-Gollop, noting the value of historical accounts even when imprecise. Some entries described “white water,” which may or may not indicate true bioluminescence. Despite this, Hudson says, the team has uncovered remarkable patterns that link milky seas to climate events, including La Niña and the Indian Ocean Dipole.
Events Spanning Entire Countries
One of the study’s most astonishing findings is the scale of these events. Some milky seas have reached sizes over 38,000 square miles—larger than Indiana—and glowed brightly enough to be seen from satellites in space. “The bigger the event, the more likely people are to sail to it,” Hudson notes, which may skew data toward more noticeable occurrences.
Most sightings cluster in the Northwest Indian Ocean and around Maritime Southeast Asia, where monsoons and oceanic upwellings provide the nutrient-rich conditions that bacteria love. Hudson explains, “These storms pull nutrients to the surface, triggering explosions in the food chain,” possibly including the glowing bacteria behind the effect.
