https://en.wikipedia.org/wiki/52-hertz_whale

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 in the north Pacific Ocean between Aleutian and Kodiak Islands to the California coast. The whale itself has never been sighted: it has only been heard via hydrophones, but its call has been detected since the late 1980s in a pattern that matches the migration of the blue whale and the fin whale. Those species call at 10 to 39 Hz and 20 Hz respectively. Described as the "world's loneliest whale", it appeared to be the only individual emitting a whale call at this frequency.