Between 1915 and 1993, 235 sightings of milky seas were documented, most of which are concentrated in the northwestern Indian Ocean near to Somalia. The luminescent glow is concentrated on the surface of the ocean and does not mix evenly throughout the water column.
In 1985, a research vessel in the Arabian Sea took water samples during milky seas. Their conclusions were that the effect was caused by the bacterium Vibrio harveyi. Mareel is typically caused by Noctiluca scintillans (popularly known as "sea sparkle"), a dinoflagellate that glows when disturbed and is found in oceans throughout much of the world.
In July 2015, at Alleppey, Kerala, India, the phenomenon occurred and the National Institute of Oceanography and Kerala Fisheries Department researched it, finding that the glittering waves were the result of Noctiluca scintillans.
In 2005, Steven Miller of the Naval Research Laboratory in Monterey, California, was able to match 1995 satellite images with a first-hand account of a merchant ship.  U.S. Defense Meteorological Satellite Program showed the milky area to be approximately  (roughly the size of Connecticut). The luminescent field was observed to glow over three consecutive nights.
While monochromatic photos make this effect appear white, Monterey Bay Aquarium Research Institute scientist Steven Haddock (an author of a milky seas effect study) has commented, "the light produced by the bacteria is actually blue, not white. It is white in the graphic because of the monochromatic sensor we used, and it can appear white to the eye because the rods in our eye (used for night vision) dont discriminate color." In Shetland (where generally caused by Noctiluca scintillans'), mareel has sometimes been described as being green, rather than the traditional blue or white milky seas effect seen by the rest of the world. It is not known whether this difference depends on the area, or simply a perception of a cyanic colour as being green.