The 52-hertz whale, colloquially referred to as 52 Blue, is an individual whale of unidentified species that calls at the unusual frequency of 52 hertz. This is a higher frequency than usual for any whale species with migration patterns most closely resembling the 52-hertz whale's: the blue whale and the fin whale (20 Hz). Its call has often been detected in many locations since the late 1980s, and it appears to be the only individual emitting a whale call at this frequency. However, the whale itself has never been sighted: it has only been heard via hydrophones. It has been described as the "world's loneliest whale", though possible recordings of a second 52-hertz whale, heard elsewhere at the same time, have been sporadically found since 2010.