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"multixrulz" writes:

A team from the National Oceanic and Atmospheric Administration, Oregon State University and the U.S. Coast Guard left a hydrophone at the bottom of the Mariana Trench [oregonstate.edu], the deepest part of the ocean, for several months to get a baseline measure of ambient noise in the ocean. This data is to be used in future studies of human-generated ocean noise. They didn't expect to hear much.

“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 scientist on the project. “Yet there really is almost constant noise from both natural and man-made sources. The ambient sound field at Challenger Deep is dominated by the sound of earthquakes, both near and far was well as the distinct moans of baleen whales and the overwhelming clamor of a category 4 typhoon that just happened to pass overhead.

“There was also a lot of noise from ship traffic, identifiable by the clear sound pattern the ship propellers make when they pass by,” added Dziak, who has a courtesy appointment in Oregon State’s College of Earth, Ocean, and Atmospheric Sciences. “Guam is very close to Challenger Deep and is a regional hub for container shipping with China and The Philippines.”

The hydrophone had enough flash to store 23 days of sound, but the device couldn't be retrieved immediately because of typhoons and shipping activity.

The researchers had never send a hydrophone so deep before: this one went 7 times deeper than any previous instrument. The pressure at that depth is nearly 1100 times air pressure at sea level. It took 6 hours to freefall to the ocean floor, because its rate was limited to 5 ms^-1 to prevent damage due to the increasing pressure. To retrieve the device, an acoustic signal was sent to tell it to release from the ocean floor and rise up on attached floats.

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