The location of the Mariana Trench's deepest point (Challenger Deep), where the hydrophone was placed. Google Earth

When you listen to the abyss, the abyss listens back to you. New information reveals artificial and natural sounds can permeate all the way to the bottom of the ocean.​

For the first time, scientists have placed a titanium-encased hydrophone on the ocean floor at the bottom of the Pacific Ocean’s Mariana Trench, located about 11,000 meters (36,000 feet) below sea level. The hydrophone recorded ambient noise for over three weeks, with the results surprising researchers. The research was carried out by the National Oceanic and Atmospheric Administration (NOAA) Pacific Marine Environmental Laboratory (PMEL).

“You would think that the deepest part of the ocean would be one of the quietest places on Earth,” Robert Dziak, a NOAA research oceanographer and chief scientist on the project, said in a statement. “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, as 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,” Dziak added.

This is the sound of a baleen whale from the microphone

The aim of this study, which comprised researchers from NOAA, Oregon State University and the U.S. Coast Guard, was to obtain precise noise measurements so that scientists can determine in the future if noise levels are increasing. However, the depths of the ocean are a particularly difficult region to study.

“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 Haru Matsumoto, who helped develop the bespoke instrument for this experiment. “We had to drop the hydrophone mooring down through the water column at no more than about five meters per second [16 feet per second]. Structures don’t like rapid change and we were afraid we would crack the ceramic housing outside the hydrophone.”

Although the task was complex, the results were clearly worth the effort. Dziak and the rest of the team have analyzed the three-weeks' worth of sounds and have separated the natural noise and the noise made from human activities. The team are now planning another expedition in 2017, where they will keep the hydrophone submerged for a longer period of time and attach a deep-ocean camera.

A ship can be heard passing overhead in this recording