Imagine roaming the world's largest ocean year after year alone, calling out with the regularity of a metronome, and hearing no response.

Such, apparently, is the situation faced by a solitary whale, species unknown, that has been tracked since 1992 in the North Pacific by a classified array of hydrophones used by the Navy to monitor enemy submarines.

The animal is called the 52 hertz whale because it 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.