...a solitary whale, species unknown, ...has been tracked since 1992 in the North Pacific...Via the New York Times, via
...[I]t makes a distinctive stream of sounds at around that basso profundo frequency, just above the lowest note on a tuba.
Its sonic signature is clearly that of a whale, but nothing like the normal voice of the giant blue or the next biggest species, the fin, or any other whale for that matter, said Mary Ann Daher, a marine biologist at the Woods Hole Oceanographic Institution on Cape Cod.
...In the current edition of the journal Deep-Sea Research, members of this team report that all signs are that the sounds come from a single animal, whose movements "appeared to be unrelated to the presence or movement of other whale species."
Team members and other experts have proposed a host of explanations for the whale sounds, among them that the animal is malformed or, most likely, is a hybrid of a blue whale and another species.
rm. Edited To Add: The journaler who brought it to rm's attention also linked to a Metafilter post with several links about the whale.