The 3.5-second symphony covers nearly half of the sound range that humans can hear, rising from a frequency as low as 38 hertz to an 8,000-hertz "metallic finale." But unless you make a detour to waters above the deepest spot in the ocean, you might never hear it. Initially stumped by the "Western Pacific Biotwang"—recorded above the deepest known part of Earth's oceans, the Mariana Trench—scientists now believe the noise comes from a species of baleen whale called minke whales. Minke whales are found in the area, but little is known about their vocalizations as the species "doesn't spend much time at the surface, has an inconspicuous blow, and often lives in areas where high seas make sighting difficult," researcher Sharon Nieukirk tells Live Science.
However, it is known that minke whales "call frequently," says Nieukirk, and acoustic robots recorded the Western Pacific Biotwang on several occasions between the fall of 2014 and spring of 2015. "It's very distinct, with all these crazy parts," Nieukirk says in a statement. "The low-frequency moaning part is typical of baleen whales, and it's that kind of twangy sound that makes it really unique." If the sound is confirmed to come from a baleen whale, it would be significant as identifying a new baleen-whale call is a rare occurrence. It's unclear what the call might mean—a mating call would only be heard seasonally—but researchers plan to collect biopsy samples to learn more, reports the Christian Science Monitor. (The Mariana Trench is quite noisy. And dirty.)