Milky seas are a rare, historically fabled form of marine bioluminescence. Characterized by their steady, non‐flashing, eponymous white glow; milky seas are capable of illuminating over 100,000 km2 ${\text{km}}^{2}$ of the nocturnal ocean surface for months at a time. Eyewitnesses have compared the experience of sailing through a milky sea to a snowy plain at night, the "Twilight Zone," and even the biblical apocalypse. Despite centuries of scientific research into milky seas very little is known about the physical and biogeochemical processes which govern their formation, longevity, and size. Scientific inquiry into milky seas has historically been held back due to the paucity of data, and the remote, ephemeral nature of the phenomenon. Through combining centuries of first‐hand eyewitness accounts with modern satellite‐based low‐light imagers such as the Day/Night Band we present the first extant database of milky sea eyewitness accounts in over 30 years. We also present the first statistical comparison between milky seas and coupled atmosphere‐ocean phenomena such as the El Niño Southern Oscillation and the Indian Ocean Dipole to elucidate connections between milky seas and potential sources of predictability within the coupled earth system. Plain Language Summary: Bioluminescence is the ability of living beings to emit light (e.g., a firefly's glow) and comes in many different shapes and forms across the natural world. Milky seas are one of the rarest and most spectacular forms of bioluminescence where at night the ocean surface will glow from horizon to horizon up to months at a time. Some milky sea events are so large and so bright they can be viewed from space using special low‐light imagers on satellites. Scientists and sailors have been fascinated by milky seas for centuries but very little is actually known about this phenomena. In this paper we combine centuries of eyewitness accounts from newspapers, diaries, ship logs, and scientific publications with satellite imagery to create the only database of milky sea sightings in over 30 years, revealing when and where milky seas occur around the world. We then show how this database can be used to better understand how global weather and ocean patterns (e.g., El Niño/La Niña) impact when and where milky seas occur. Key Points: A global database of milky sea observations is presented, primarily from eyewitness accounts derived from ship logs spanning 400 yearsMilky sea sightings occur primarily in the Northwest Indian Ocean (62%) and in the Maritime Continent Region (19%)The timing of milky sea sightings are statistically shown to be related to the Indian Ocean Dipole and the El Niño Southern Oscillation [ABSTRACT FROM AUTHOR]

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