“There was also a lot of noise from ship traffic, identifiable by the clear sound pattern the ship propellers make when they pass by,” he added.

Researchers from NOAA and Oregon State University worked with the US Coast Guard to conduct the experiment, which was designed to establish a baseline level of noise at the bottom of the Mariana Trench. Future scientists will be able to repeat the process to establish the extent to which manmade noise is increasing.

In order to make the recording, the scientists had to take into account the massive atmospheric pressure that builds at such depth.

“We had never put a hydrophone deeper than a mile or so below the surface, so putting an instrument down some seven miles into the ocean was daunting,” said Oregon State engineer Haru Matsumoto. “We had to drop the hydrophone mooring down through the water column at no more than about five meters per second. Structures don’t like rapid change and we were afraid we would crack the ceramic housing outside the hydrophone.”

It took more than six hours for the hydrophone to reach the bottom in July 2015 and, after recording sound for 23 days, the researchers had to wait until November for passing ships and typhoons to clear so that they could retrieve the device.

Having successfully recorded sound at this depth, an Oregon State University researcher is planning to return to the Mariana Trench in 2017—this time, with a deep-ocean camera.

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