For centuries, sailors have reported the eerie glow of “milky seas” — vast areas of ocean that shine at night like a soft green-white light.
Scientists now believe this rare phenomenon is caused by bioluminescent bacteria, but much remains unknown. Researchers at Colorado State University have created a 400-year database to help track when and where these glowing seas appear, linking them to large-scale climate events like El Niño. The hope is to one day study a milky sea in real-time and uncover its ecological role—whether it signals ocean health or harm—while unraveling how these glowing patches connect Earth’s surface, sky, and biosphere.
Mysterious Ocean Glow Puzzles Scientists
For generations, sailors around the world have reported a strange and beautiful sight: large areas of the ocean glowing steadily at night, sometimes for weeks or even months. The glow is bright enough to read by and resembles the soft greenish-white light of glow-in-the-dark stars often found in children’s bedrooms. These glowing patches of ocean can cover up to 100,000 square kilometers, an area nearly the size of Iceland, and are sometimes visible from space.
This rare phenomenon is known as a “milky sea,” a term coined by sailors who experienced it firsthand. Although reports date back hundreds of years, milky seas remain poorly understood due to their rarity and remote locations, most often in the Indian Ocean, far from regular observation. Scientists believe the glow is likely caused by bioluminescent bacteria, particularly a species called Vibrio harveyi.
A 400-Year Archive of Glowing Encounters
To improve understanding and prediction of these events, researchers at Colorado State University have assembled a database of sightings spanning the past 400 years. Published in the journal Earth and Space Science, the archive combines historical sailor accounts, records from the Marine Observer Journal over an 80-year period, and modern satellite observations. It’s the most comprehensive dataset on milky seas in three decades. The data reveal that most sightings occur near the Arabian Sea and Southeast Asia and appear to be linked to large-scale climate patterns such as the Indian Ocean Dipole and the El Niño Southern Oscillation.
Both of those climate phenomena are known to impact global weather patterns, prompting researchers to wonder how the dazzling phenomenon may be connected to those processes.
Toward Real-Time Discovery
Justin Hudson, a Ph.D. student and the paper’s first author, said the database will help researchers better anticipate when and where a milky sea will occur. The goal, he said, is to get a research vessel out to the site in time to collect information about the biology and chemistry within a milky sea. Information about those variables could be helpful to connecting the event to broader Earth systems activity.
Hudson added that the regions where milky seas occur feature a lot of biological diversity and are important economically to fishing operations.
“It is really hard to study something if you have no data about it,” Hudson said. “To this point, there is only one known photograph at sea level that came from a chance encounter by a yacht in 2019. So, there is a lot left to learn about how and why this happens and what the impacts are to those areas that experience this.”
What Makes the Seas Glow?
Bioluminescence comes in many forms across nature. One of the most common examples is a firefly’s flickering taillight. With milky seas, though, researchers are still trying to understand what is actually happening at the sea surface.
One hint comes from a research vessel that had a chance encounter with a milky sea in 1985, which was able to collect a water sample. Researchers found that a specific strain of luminous bacteria was living on the surface of algae within a bloom – possibly causing an even glow in all directions. However, that is just one data point and could be misleading. To bridge the gap in understanding, researchers have tried to leverage information gathered from sporadic satellite imagery.
Ocean Currents and the Bacterial Cycle
Hudson said that, because of the regions most associated with the phenomenon, it seems likely the light is due to a biological process related to the bacteria.
“The regions where this happens the most are around the northwest Indian Ocean near Somalia and Socotra, Yemen, with nearly 60% of all known events occurring there. At the same time, we know the Indian monsoon’s phases drive biological activity in the region through changes in wind patterns and currents,” he said. “It seems possible that milky seas represent an understudied aspect of the large-scale movement of carbon and nutrients through the Earth system. That seems particularly likely as we learn more and more about bacteria playing a key role in the global carbon cycle both on land and in the ocean.”
Linking the Microscopic to the Global
Professor Steven Miller is the other author on the paper and has been conducting research on milky seas for years at CSU. He has led efforts to image the phenomenon from satellites and said the database should present new opportunity to get first-hand knowledge.
“Milky seas are incredible expressions of our biosphere whose significance in nature we have not yet fully determined,” Miller said. “Their very existence points to unexplored connections between the surface and the sky, and between microscopic to the global scale roles of bacteria in the Earth system. With the help of this new database, forged from sea-faring ships of the 17th century all the way to spaceships of modern times, we begin to build a bridge from folklore to scientific understanding.”
Future Research and Open Questions
The paper represents a portion of Hudson’s research at CSU in the Department of Atmospheric Science as he works to finish and defend his thesis on the subject this summer. He said he hopes the database will further illuminate our understanding of the phenomenon.
“We have no idea what milky seas mean for the ecosystems they are found in. They could be an indication of a healthy ecosystem or distressed one – the bacteria we suspect are behind it are a known pest that can negatively impact fish and crustaceans,” he said. “Having this data ready allows us to begin answering questions about milky seas beyond hoping and praying a ship runs into one accidentally.”
Reference: “From Sailors to Satellites: A Curated Database of Bioluminescent Milky Seas Spanning 1600-Present” by J. Hudson and S. D. Miller, 9 April 2025, Earth and Space Science.
DOI: 10.1029/2024EA004082
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