An ambitious scientific mission has discovered that the deepest known point in the ocean turns out to be a far noisier place than might be expected

If Mount Everest was rotated upside down and pointed into the Mariana Trench, the summit would still be a mile away from the deepest point of this underwater canyon. At 2,550km long and up to a record 11km deep at the iconic ‘Challenger Deep’, the Mariana Trench, located roughly 200km east of the Mariana Islands, is one of the furthest points on Earth away from civilisation.

You might, think, therefore, that it would be a relatively quiet place, and certainly that is what marine scientists from the National Oceanic and Atmospheric Administration (NOAA) at Oregon State University expected to find. ‘Yet there really is almost constant noise from both natural and man-made sources,’ explains Robert Dziak, a NOAA research oceanographer and chief scientist on a project to drop a titanium-encased hydrophone into the trench, which he describes as akin to ‘sending a deep-space probe to the outer solar system’.

‘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 tasked with creating a listening device capable of withstanding 36,000 feet-worth of oceanic pressure bearing down on it. That’s roughly 16,000 pounds per square inch (PSI), or more then 1,000 times more than regular atmospheric pressure. ‘We had to drop the mooring down through the water column at no more than about five metres per second,’ continues Matsumoto. ‘Structures don’t like rapid change and we were afraid we would crack the ceramic housing outside the hydrophone.’

3D perspective of bathymetry at Challenger Deep, Mariana Trench (Image: Center for Coastal and Ocean Mapping - Joint Hydrographic Center)

The purpose of the project was to establish a baseline for ambient sound in the ocean, in order to allow for future monitoring of sounds from maritime human activity. However, what they found was considerably more noise in the trench than was anticipated. ‘The ambient sound field at Challenger Deep is dominated by the sound of earthquakes, both near and far,’ reveals Dziak, ‘as 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.’

The range and magnitude of sounds recorded during the 23 days when the hydrophone was recording in the trench emphasises the strength which sound waves maintain over long distances underwater, with the hydrophone even picking up the sounds of waves seven miles above. ‘We recorded a loud magnitude 5.0 earthquake that took place at a depth of about 10km, or more than six miles, in the nearby ocean crust,’ says Dziak. ‘Since our hydrophone was at 11km, it was actually below the earthquake, which is really an unusual experience.’

There have famously only been two manned descents to the bottom of the trench – oceanographers Don Walsh and Jacques Piccard in 1960, and film director James Cameron in 2012 – however recent research projects such as this by the NOAA are opening the ocean up for new exploration. A follow-up mission aims to return the hydrophone – accompanied by a deep-sea camera – to the Mariana Trench next year.