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Historical accounts of vast ocean waters glowing in the dark go back hundreds of years, and researchers are still trying to determine exactly what triggers the phenomenon
April 15, 2025 9:05 a.m.
On a night in 1854, the crew of the American clipper ship Shooting Star experienced an unforgettable sight off the coast of Java, Indonesia. The captain’s account captures the eerie beauty: “The whole appearance of the ocean was like a plain covered with snow. There was scarce a cloud in the heavens, yet the sky… appeared as black as if a storm was raging. The scene was one of awful grandeur—the sea having turned to phosphorus, and the heavens being hung in blackness.” His vivid description illustrates the captivating yet unsettling nature of the phenomenon known as “milky seas.”
This rare bioluminescent occurrence lights up vast expanses of ocean in brilliant greens and blues for extended periods, often perplexing those who witness it. Such accounts detail a natural spectacle that has intrigued seafarers throughout history.
Current research suggests that bioluminescent bacteria may be responsible for this phenomenon. “We strongly suspect—based on the characteristics of the light and how long it lasts—that it is bacteria,” explains Justin Hudson, an atmospheric scientist at Colorado State University. Despite this insight, questions remain about how these microscopic organisms can collectively create a glow visible from extensive distances.
Studying milky seas presents significant challenges due to their unpredictable nature. Only a single photograph of the phenomenon from sea level exists, taken in 2019. However, Hudson and his colleague Steven Miller have made significant progress by creating a comprehensive database of sightings spanning four centuries. This compilation includes firsthand accounts, records from the Marine Observer journal, and satellite observations, identifying 415 occurrences of the milky seas phenomenon.
Abigail McQuatters-Gollop, a plankton ecologist at the University of Plymouth, notes that testimonies from non-scientists add a unique perspective, revealing that their observations may not have been intended for ecological analysis. Still, she commends Hudson and Miller for their effective use of the available data.
As detailed in a recent article published in the journal Earth and Space Science, the research indicates that most sightings of milky seas occur in specific regions, particularly the Arabian Sea and south of Java in the Banda Sea. It also correlates these occurrences with two significant weather patterns: the Indian Ocean Dipole and the El Niño Southern Oscillation.
In a notable 1985 event, researchers managed to collect a water sample during a milky seas incident, which contained the luminous bacterium Vibrio harveyi. This provides a possible biological explanation for the phenomenon, suggesting that it is indeed linked to the activities of these microorganisms.
The regions where milky seas are frequently observed often experience upwelling, a process that brings nutrient-rich water to the ocean’s surface, fostering marine life. Hudson emphasizes that such areas are conducive to biological activity, but many regions exhibit similar characteristics, leaving scientists pondering what distinguishes sites where milky seas occur from others.
An additional area of inquiry revolves around the ecological implications of milky seas. Researchers are interested in determining whether these glowing waters signify a healthy marine environment or indicate underlying issues. Hudson articulates hope that the new database will facilitate a deeper understanding of milky seas and support scientific efforts beyond serendipitous encounters at sea.
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