For centuries, sailors have told tales of milky seas – huge swaths of ocean glowing on dark nights, seen in blue in this false–color satellite image. Steven D. Miller/NOAA (Credit: Colorado State University/NOAA)
In a nutshell
- Milky seas are real, massive bioluminescent events that are sometimes larger than 100,000 square kilometers. They can glow for weeks and are now detectable by satellite.
- The glow is caused by dense populations of bioluminescent bacteria, likely Vibrio harveyi, which emit light through a process called quorum sensing when their numbers reach critical levels.
- These glowing seas are not random, they cluster in specific regions and seasons, especially during monsoons, and are linked to ocean-atmosphere patterns like the Indian Ocean Dipole and La Niña events.
FORT COLLINS, Colo. — For centuries, sailors have returned from voyages with tales of vast stretches of ocean that glow white at night, creating an eerie illumination that stretches from horizon to horizon. These “milky seas,” as they’re called, were often dismissed as tall tales or hallucinations of exhausted mariners. But these phenomena are very real, and they’re now being tracked from space.
According to eyewitness accounts documented in a new comprehensive study published in Earth and Space Science, sailors throughout history have described these encounters in dramatic terms, comparing the experience to being in the “Twilight Zone,” witnessing the biblical apocalypse, or seeing the ocean haunted by spirits.
These legendary glowing waters aren’t small flashes of light like the more common bioluminescence created when boats disturb certain plankton. Instead, milky seas can illuminate over 100,000 square kilometers of ocean surface, an area larger than South Korea, and persist for weeks or even months.
Scientists Justin Hudson and Steven D. Miller from Colorado State University have compiled the first comprehensive database of these events in over 30 years, collecting 415 observations dating back to the 1600s. Their research reveals that these massive light shows aren’t random but follow distinct patterns tied to global weather systems and ocean conditions.
“It is really hard to study something if you have no data about it,” says Hudson, in a statement. “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.”
Understanding how milky seas function within the global carbon cycle and what they reveal about ocean ecosystem health is an important step toward comprehending how oceans may evolve in our changing climate.
The glowing phenomenon likely comes from bioluminescent bacteria that colonize algae. These bacteria emit light through a population-driven process called quorum sensing, where the organisms essentially “decide” to glow once their population reaches a critical density. To create a milky sea visible to the human eye, bacterial densities on the order of 10 million to 1 billion cells per milliliter are needed, which is an extraordinary concentration considering these bacteria typically exist in background levels of just 10 cells per milliliter in ocean water.
Most eyewitness accounts describe the sea being illuminated with a steady, non-flashing glow that’s white, gray, or greenish-blue. Unlike the brief flashes of light caused by disturbed dinoflagellates (the common form of marine bioluminescence), milky seas create a continuous glow that can persist for nights on end.
The paper includes a documented account from 1967, where a second officer on the SS Ixion observed the sea glowing “from horizon to horizon in all directions” with what he described as “a definite glow similar to the glow of a luminous watch.” He reported difficulty focusing his eyes when looking into the water and experiencing “a slight feeling of vertigo,” noting the experience was disorienting enough that it could have convinced superstitious sailors of earlier eras they might “fall off the edge of the world.”
The researchers found these glowing seas occur primarily in three global hotspots: the Northwest Indian Ocean (accounting for 62% of observations), the waters south of Java, Indonesia (11%), and the Banda Sea (7%). The timing isn’t random either. Milky seas in the Northwest Indian Ocean peak during the boreal summer monsoon (July-September) and winter monsoon (January-February).
This seasonal pattern, combined with deep statistical analysis, led researchers to discover that milky seas are often associated with specific ocean-atmosphere conditions. For instance, waters south of Java are significantly more likely to develop milky seas during a positive phase of the Indian Ocean Dipole, a climate pattern in which the western Indian Ocean becomes warmer than average while the eastern portion cools.
Modern satellite technology has revolutionized how we study these elusive phenomena. Low-light imaging sensors on satellites can now detect the faint glow emitted by milky seas. The Visible/Infrared Imaging Radiometer Suite Day/Night Band can detect light emissions about 10 million times fainter than daytime reflected sunlight.
These satellite observations have confirmed what sailors have reported for centuries. Some milky seas observed from space have persisted for at least 45 days and reached sizes exceeding 100,000 square kilometers.
Milky seas tend to form in cooler, upwelled waters. Sea surface temperature data showed that they occur predominantly in the cooler half of temperature distributions, suggesting they’re associated with nutrient-rich upwelled waters, an important clue to their formation.
The researchers believe that monsoons and other weather patterns create conditions that lead to massive algal blooms, which then become colonized by bioluminescent bacteria. The bacteria may either form a symbiotic relationship with healthy algae or feed on dying algal blooms, essentially creating a massive bacterial light show visible from both ships and satellites.
These glowing waters could provide insights into ocean health and how marine ecosystems might respond to climate change. The database compiled by the researchers is a remarkable collection of maritime history spanning four centuries. From sailors’ logbooks to modern satellite imagery, it chronicles humanity’s long fascination with this ethereal ocean phenomenon that blurs the line between myth and science.
By unlocking the mystery of milky seas, scientists are gaining new insight into the complex interactions between weather, ocean conditions, and the microbial life that illuminates the seas. The research shows that these vast glowing waters are not random but predictable events tied to global climate patterns—and now, for the first time, we may be able to forecast where and when the oceans will next light up the night.
Paper Summary
Methodology
The researchers constructed a database of 415 milky sea observations spanning from the 1600s to present day. They collected eyewitness accounts from various sources including the Marine Observer journal (published by the UK Meteorological Office from 1924-2003), unpublished notes from bioluminescence experts, scientific literature, and modern satellite observations from the Visible/Infrared Imaging Radiometer Suite Day/Night Band. The team classified accounts based on confidence levels (high, low, or very low) depending on how well the descriptions matched known characteristics of milky seas. They then analyzed these observations against atmospheric and oceanic data including the Indian Ocean Dipole, El Niño Southern Oscillation, and monsoon indexes to identify potential correlations and predictors for milky sea formation.
Results
The study found that milky seas primarily occur in three global hotspots: the Northwest Indian Ocean (62% of observations), waters south of Java (11%), and the Banda Sea (7%). The timing of these events follows seasonal patterns aligned with monsoon cycles. Statistical analysis revealed significant correlations between milky sea occurrences and specific climate patterns. For example, milky seas south of Java were strongly associated with positive phases of the Indian Ocean Dipole, while Northwest Indian Ocean events during boreal summer were more likely following La Niña events. The researchers also determined that milky seas typically last less than three weeks and cover areas of approximately 10,000 square kilometers, though satellite observations have detected some lasting 45+ days and covering more than 100,000 square kilometers. Sea surface temperature data showed milky seas occur predominantly in cooler upwelled waters.
Limitations
The database has several sources of bias. Most observations come from shipping routes, so areas with less maritime traffic may have undetected events. There’s temporal bias with 65.6% of accounts occurring between 1950-2000, reflecting the publication timeline of the Marine Observer journal. Observer training and language barriers also create biases, as most accounts come from English-speaking sailors with limited scientific training. Satellite detection is limited to moonless nights with minimal cloud cover, biasing those observations toward larger, brighter events. The study also lacks input from local communities living around the Indian Ocean, potentially missing valuable historical observations.
Funding/Disclosures
The research was supported by the NOAA Joint Polar Satellite System Program Office under Award Numbers NA19OAR4320073 and NA24OARX432C0007.
Publication Information
The paper titled “From Sailors to Satellites: A Curated Database of Bioluminescent Milky Seas Spanning 1600-Present” was authored by Justin Hudson and Steven D. Miller from Colorado State University and the Cooperative Institute for Research in the Atmosphere. It was published in Earth and Space Science in 2025.
