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.
…
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.
(NYT: “Song of the Sea, a Cappella and Unanswered,” by Andrew Revkin [December 21, 2004])