Seven miles below the surface of the sea, the Challenger Deep is the deepest spot in the Mariana Trench and thus on Earth. The only light is that produced by animals that use photoluminescence; no sunlight can penetrate even close to these waters. Such life as exists is either extremely small, extremely slow-moving, or both.
So the deepest place on Earth could reasonably be assumed to be almost deathly quiet.
And yet, according to new NOAA research, it is not.
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That the ocean as a whole is full of sound is not a new observation: from pistol shrimps to Weddell seals, walruses to blue whales, the cracking of icebergs and the rumbling of earthquakes, Nature's underwater symphony is varied and at times cacophonous. But those harmonies have in recent decades been joined by ships' propellers, pile driving and the loud booms of seismic air guns searching for oil and gas deposits under the sea bed.
In order to get a better handle on the extent of these new additions, scientists from NOAA and Oregon State University have begun a survey to measure noise in the ocean. As a first step, they wanted to establish a baseline, and to do so, they elected to lower a hydrophone into the Challenger Deep.
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The purpose-built hydrophone was crafted from titanium nearly an inch thick to withstand pressures of 16,000 pounds per square inch. It was lowered to the seafloor over a six-hour period from the deck of the US Coast Guard cutter Sequoia; a proprietary anchor system held it in place, and the researchers departed, returning several months later to pick up the recorder, sending an acoustic signal that triggered a release mechanism and allowed the device to rise slowly to the surface.
Expecting to hear very little, they were surprised by what they had captured.
"You would think that the deepest part of the ocean would be one of the quietest places on Earth," said Robert Dziak, a NOAA research oceanographer and chief project scientist, in a press release.
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"Yet there is almost constant noise. The ambient sound field is dominated by the sound of earthquakes, both near and far, as well as distinct moans of baleen whales, and the clamor of a category 4 typhoon that just happened to pass overhead."
The extent to which marine mammal vocalizations were captured was unexpected. Few whales dive much deeper than about a mile - yet their voices traveled clearly to the bottom of the trench. Yet even at this remote of depths, the hydrophone also picked up sound from ship propellers. Challenger Deep is close to Guam, a regional hub for container shipping with China and the Philippines.
Human-created noise has increased steadily in recent decades and getting these first recordings allows scientists in the future to determine if the noise levels are growing and how this might affect marine animals that use sound to communicate, navigate and feed, such as whales, dolphins and fish.
Researchers plan to return in early 2017, to deploy the hydrophone for a longer period of time and attach a deep-ocean camera.