Looking for a silent spot to get away from it all? You might think the deepest ocean is a good contender, but that’s definitely not the case.
For three weeks, a titanium-encased sensor called a hydrophone recorded ambient noise from the bottom of the Challenger Deep, a trough in the Mariana Trench 11 kilometres deep. And rather than a sea of silence punctuated by the odd whale call, the hydrophone picked up a cacophony of sounds, including the rumbles of an earthquake and passing ships overhead.
The project was designed to obtain a baseline measurement for noise under the waves. But when they listened to the recording, the researchers – from Oregon State University, the National Oceanic and Atmospheric Administration and the US Coast Guard – were shocked by the racket.
“You would think that the deepest part of the ocean would be one of the quietest places on Earth,” says Robert Dziak, oceanographer at the National Oceanic and Atmospheric Administration and project lead.
“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 clamour 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.”
Challenger Deep sits near the island nation Guam, which is is a regional hub for container shipping with China and the Philippines.
Building the hydrophone was a feat in itself. In the deepest ocean, the pressure is enormous. Normal room air pressure measures one atmosphere of pressure; at the bottom of Challenger Deep, the pressure is a crushing 1,089 atmospheres, or 16,000 psi.
So the hydrophone sensor needed to be able to withstand the crush. And when they plopped the hydrophone into Challenger Deep in July last year, they had to do so slowly, otherwise the ceramic housing might crack. It took six hours to sink to the bottom.
For next 23 days the hydrophone recorded sounds, storing them on its flash drive housed within, but shipping schedules and typhoons meant the team couldn’t drag it back up until November.
Over the months since, Dziak and his crew listened and analysed the recordings. From the constant burble, they were able to identify ship propellers – as well as a magnitude 5.0 earthquake, which took place at 10 kilometres deep in the crust.
“Since our hydrophone was at 11 kilometres, it actually was below the earthquake, which is really an unusual experience,” Dziak says.