Many may assume that the deepest part of the ocean is the most silent place in the world. However, a team of researchers from the National Oceanic and Atmospheric Administration (NOAA), Oregon State University and the US Coast Guard have recorded both natural and anthropogenic or human-caused noise in the Mariana Trench.
“The ambient sound field at Challenger Deep (Mariana Trench’s lowest point) is dominated by the sound of earthquakes (5.0), 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,” says chief scientist Robert Dziak, an NOAA research oceanographer.
“There was also a lot of noise from ship traffic, identifiable by the clear sound pattern the ship propellers make when they pass by. Guam is very close to Challenger Deep and is a regional hub for container shipping with China and The Philippines,” Dziak adds.
In July 2015, the researchers deployed their titanium-encased hydrophone to record the ambient noise in the Challenger Deep, more than 36,000 feet (10.9 km) below the ocean’s surface. This hydrophone withstood the Mariana Trench’s atmospheric pressure at 16,000 pounds per square inch, which is only about 14.7 PSI at home or office.
“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,” says Haru Matsumoto, an Oregon State ocean engineer. “We had to drop the hydrophone mooring down through the water column at no more than about five metres per second. Structures don’t like rapid change and we were afraid we would crack the ceramic housing outside the hydrophone.”
The hydrophone took more than six hours to free fall to the bottom and recorded every sound for 23 days deep in the Trench. The team retrieved the hydrophone in November 2015 by sending an acoustic signal that released it from the seafloor.
Dziak said that this was kind of like exploring the outer solar system. The recording will help researchers determine if noise levels are growing.
The researchers plan a return mission to the Challenger Deep in 2017 that will be headed by Oregon State University co-investigator, Joe Haxel. This time, the hydrophone will be attached with a deep-ocean camera and will stay there for longer period of time.