For centuries, a mysterious phenomenon has circulated among sailors. It is said that on a moonless night, the sea shines bright white, giving the illusion that a ship is floating on the Milky Way. This phenomenon is so surreal that it has inspired works like IAAN director's film "Life of Pi" and Jules Verne's novel "Twenty Thousand Leagues Under the Sea." Sailors and scientists have named this phenomenon "Milky seas," as it resembles the Milky Way that adorns the night sky. There is still no Korean term for this phenomenon.

The light that spreads over an expansive sea area of 100,000 square kilometers is even detectable from space. American scientists have found a method to track the rare luminescent phenomenon in the sea using information left by sailors, scientists, and satellites for nearly 400 years.

Almost imperceptible luminescent phenomenon in seawater

The phenomenon of seawater glowing white at night, known as the Milky sea, is so surreal that it features in works of art such as director IAAN's film Life of Pi and Jules Verne's novel Twenty Thousand Leagues Under the Sea. The light spread across an area of 100,000㎢ can even be seen from space. /Courtesy of 20th Century Fox

Research teams at Colorado State University reported on the 9th (local time) that they have built a database using sailors' testimonies, navigation records, and 30 years of satellite observation data to accurately predict the locations and times of the luminescent phenomenon known as "Milky seas" that occurs at night.

Milky seas are a type of bioluminescent phenomenon in which marine organisms such as bacteria emit light extensively at the sea surface at night. The light is bright enough to read a book by and can last anywhere from a few days to several months. This rare phenomenon has been witnessed by sailors navigating distant seas for centuries, but it was mainly reported as word of mouth. It is still observed very rarely today, so little is known about its exact causes. Milky seas have mostly been observed in remote areas of the Indian Ocean, where they are not easily seen by the human eye, occurring only 1 to 2 times a year.

The research team has built a large-scale archive using 235 accounts of sailors' sightings reported over 400 years, testimonies submitted to the Marine Observer Journal over the past 80 years, and satellite data collected over the last 30 years. The findings suggest that Milky seas mostly occur in the Arabian Sea and Southeast Asian waters. The area where this phenomenon has been most frequently reported to date is the northwestern waters of the Indian Ocean near Socotra Island off the southern coast of Somalia and Yemen. So far, 60% of recorded or reported instances of Milky seas have occurred in this area.

This phenomenon has also been statistically linked to the Indian Ocean dipole and the El Niño Southern Oscillation (ENSO). The Indian Ocean dipole is a phenomenon where the temperatures in the eastern and western Indian Ocean change. When the eastern Indian Ocean shows anomalous warming and the western Indian Ocean shows anomalous cooling, it has a positive value; conversely, it has a negative value under opposite conditions. The El Niño Southern Oscillation refers to the circulation between the El Niño phase, where sea temperatures in the equatorial eastern Pacific are above average, and the La Niña phase, where they are below average. Both climatic phenomena are known to impact global weather patterns. Justin Hudson, the lead author of the paper, noted, "The periodicity of the Indian Ocean monsoon seems to promote biological activity in this region through changes in wind patterns and ocean currents."

Mysterious phenomenon in the Indian Ocean and Arabian Sea

In July 1985, American researchers discovered bioluminescence in the Arabian Sea and collected samples. They found bacteria called Vibrio harveyi in the samples, which thrive attached to algae. /Courtesy of S. Haddock·MBARI

Bioluminescence occurs in various forms in nature outside of the ocean. The flickering tail of fireflies is a prime example. Green fluorescence is also often observed in the ocean. Researchers have long sought to understand what happens in the white luminescent phenomenon of Milky seas. However, the amount of concrete information gathered so far is woefully inadequate. Hudson stated, "So far, the only photograph of Milky seas was taken accidentally by a yacht in 2019," adding that "much work remains to uncover how and why this phenomenon occurs and what effects it has on the ocean."

In July 1985, American researchers discovered bioluminescence in the Arabian Sea and collected samples. They found bacteria called Vibrio harveyi in the samples, which they discovered live attached to algae where they thrive. Scientists estimate that this microorganism emits light when stimulated by interactions with specific algae and phytoplankton communities.

However, scientists assess that these results are merely one set of data and do not present sufficient evidence. They are tracking how climate affects Milky seas. More information is needed to link the bioluminescent phenomenon to broader Earth system activities. Hudson stated, "This database is expected to help scientists predict when and where Milky seas phenomena will occur," adding that "the goal is to deploy research vessels in a timely manner to collect information on the biological and chemical processes occurring in Milky seas."

Recently, scientists have focused on information collected by satellites to fill in the gaps in available information. Professor Steven Miller, who participated in this research, has been exploring ways to capture Milky seas phenomena using satellites for over 20 years. The research team began their study after reports in 1995 that the British merchant ship Lima encountered Milky seas while sailing northwest off the coast of Somalia. In the early 2000s, Professor Miller, who was working at the U.S. Naval Research Laboratory, conducted research to capture luminescent phenomena at sea using the Visible Infrared Imaging Radiometer Suite (VIIRS) being developed by the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA). The team reported observing luminescent phenomena over an area of about 15,400 square kilometers in the northwestern Indian Ocean for three consecutive days using a U.S. Department of Defense satellite. This was the first time Milky seas were captured by satellite. The research team presented their findings in 2021 in the journal Scientific Reports, stating that they discovered 12 instances of luminescence across the Indian and western Pacific Oceans based on data captured by the Suomi NPP satellite from 2012 to 2021.

Tracking 400 years of sailors' testimonies, navigation records, and satellite data

Professor Steven Miller's research team from Colorado State University succeeded in capturing a Milky sea covering approximately 100,000㎢ using satellites based on photographs taken by the crew of the superyacht Ganesha near Java Island, Indonesia, in 2019, published in the Proceedings of the National Academy of Sciences (PNAS). /Courtesy of PNAS

The research team believes that the information captured by satellites will provide new opportunities to gain more diverse knowledge about Milky seas. Professor Miller stated, "While Milky seas are an astonishing phenomenon found in nature, their role is still not clarified," adding that "it illustrates how small microorganisms like bacteria affect Earth as a whole, including the land and sky."

Scientists are unable to assess how Milky seas impact the overall ecosystem. The luminescent phenomena found in the ocean could indicate that the ecosystem is healthy or conversely, may be signs of an ecosystem in decline. In fact, the bacteria emitting light in Milky seas are classified as harmful to fish and crustaceans.

The research team considers that Milky seas may correspond to still unknown areas in the large-scale carbon and nutrient cycles occurring on Earth. As progress is made in studying bacteria that play a key role in carbon cycling on land and in the ocean, more of their secrets are being unveiled.

The areas reported for the Milky seas phenomenon coincide with regions rich in biodiversity and active fishing activities. The research team stated, "By utilizing information collected from 17th-century vessels to modern satellites, we have begun to build a bridge connecting sailors' stories with science," adding that "we expect to gain much more answers than merely wishing to accidentally encounter Milky seas."

The geographical distribution of the brightly shining sea at night. They represented shading according to observation counts, dividing latitude and longitude by 15 degrees (above). The research team also marked three major hotspots suitable for observation (black boxes in the lower image). /Courtesy of Colorado State University

References

Earth and Space Science (2025), DOI: https://doi.org/10.1029/2024EA004082

Journal of Experimental Marine Biology and Ecology (1988), DOI: https://doi.org/10.1016/0022-0981(88)90152-9

Scientific Reports (2021), DOI: https://doi.org/10.1038/s41598-021-94823-z