52 Hz sound - blue whale call..
http://www.pmel.noaa.gov/vents/acoustics/whales/sounds/whalewav/ak52_10x.wav

For decades now, scientists at the NOAA have been tracking a mysterious whale song that sounds like the ghostly howls of a drowned tuba player. The sounds have been identified as belonging to a single whale, who sings at a frequency unlike any other whale in the world.
Dubbed "52 Hertz" after the frequency range in which he typically sings, the animal has been called the loneliest whale in the world, since his love songs seem destined to go unanswered. Most other species of baleen whale, such as blue whales and humpbacks, sing at frequencies much lower, between the 15-25 Hertz range.

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http://blogs.discovery.com/animal_news/2012/05/52-hertz-the-loneliest-whale-in-the-world.html