A Different Kind of ‘Milk Run’

Many sailors have seen the elusive green flash, but we wonder how many have seen an (apparently) even rarer event: the so-called “milk sea” or “milky sea” that emits an intense glow at night. This is not to be confused with ‘regular’ bioluminescence (also known by the common but technically incorrect term “phosphorescence”), in which planktonic organisms can be excited to light up at night by boat wakes or waves breaking on the beach. In a milk sea, the whole ocean glows, often for hundreds of miles.

Sailors have been reporting this phenomenon for centuries — Jules Verne made accurate mention of it in his 1869 novel 20,000 Leagues Under the Sea — but it wasn’t until 2005 that satellite images were discovered to have recorded such an event in the Indian Ocean off Somalia. In 1995, an area about the size of Connecticut glowed for three nights in a row. A ship transiting the area confirmed the miles of glowing water. That was just one of 235 documented sightings of milk seas since 1915. Most — but not all — were reported in the Indian Ocean and near Indonesia.

Scientists are so far at a loss to explain the phenomenon. One hypothesis is that milk seas may be caused by biolumescent bacteria reacting with something else. “The problem with the bacteria hypothesis is that an extremely high concentration of bacteria must exist before they begin to produce light,” says Steven Miller, the Naval Research Laboratory scientist who led the space-based discovery. What could cause the massive blooms of bacteria — and what they could possibly react with to form a milk sea — remain a mystery.

Have any of you readers encountered encountered this type of ‘milk run’? We’d love to hear about it.