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.
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The 52 hertz whale may be maturing, since its voice has deepened slightly over time, Ms. Daher said. A gallery of sounds, including the call of 52 hertz, can be heard at www.pmel.noaa.gov/vents/acoustics/spectrograms.html.

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.