For many years, scientists have been using a Cold War-era network of underwater microphones in the North Pacific to track baleen whale migrations which occur between August and December.
One whale always stands out, however: the "Christmas Whale."
Since 1989, researchers have been tracking this specific Blue Whale based on its distinct vocalizations.
Baleen whales have poor eyesight and cannot smell much underwater, so sounds are how they recognize one another, navigate, and find mates. But these vocalizations happen in very specific frequency ranges, depending on the species — between 10 and 31 hertz. The Christmas Whale, however, vocalizes at 52 hertz, which has been described as "bringing a piccolo to a tuba party."
Over the last 25 years, the Christmas Whale -- 52 Blue -- has grown and matured, based on its voice deepening slightly, but it also seems to exist outside of any normal whale social system.
It travels alone, and its high-pitched pleas are never answered.
One possibility is that the mystery whale is a hybrid between two different species, though the pattern of behavior seems to indicate a Blue Whale. There are no other species of whale that could possibly cross with a Blue Whale, which is the largest animal that has ever existed in the history of the world.