SEATTLE — The deepest spot on Earth is a surprisingly noisy place, scientists from Oregon discovered when they lowered a hydrophone about 11 kilometres below the ocean surface into the Challenger Deep.
Left in place for several months, the device recorded the booming cries of whales, the rumble of ships passing overhead and the crescendos from earthquakes deep in the planet’s crust.
“This should be one of the quietest places in the world, but it was a lot noisier than we expected,” said Oregon-based oceanographer Robert Dziak, who led the project for the National Oceanic and Atmospheric Administration. “There really is almost constant sound from natural and man-made sources.”
Only a handful of vessels have ever penetrated the Challenger Deep, the deepest canyon in the fabled Mariana Trench near Micronesia. If Mount Everest were tucked into the nearly 11,000-metre chasm, it would still be covered by about 1,500 metres of water.
The most recent human visitor was filmmaker James Cameron, who descended to the bottom alone in 2012 and spent two hours there in his custom-built submarine.
A few remotely operated vehicles and instruments have been deployed in the Mariana Trench, but Dziak said the recent recordings might be the first-ever captured at such great depths.
“It’s not an easy thing to do, to get an instrument package to the sea floor, recover it from that depth and have it survive,” he said.
The 50-centimtre-long hydrophone was designed for the job by Oregon State University engineer Haru Matsumoto and Chris Meinig, of NOAA’s Pacific Marine Environmental Laboratory in Seattle. It was crafted from titanium about two centimetres thick to withstand pressures of 16,000 pounds per square inch — a force that could crush a car like paper crumpled in a fist.
“We had never put a hydrophone deeper than a mile or so below the surface,” Matsumoto said.
To avoid rapid pressure change, the device was lowered to the sea floor over a six-hour period. An anchor system also developed at the marine laboratory held the instrument in place, suspended about six metres above the soft bottom.
The U.S. Coast Guard volunteered the Guam-based cutter Sequoia for the deployment mission in July 2015.
The scientists returned to fetch the recorder in November, sending an acoustic signal that triggered a release mechanism and allowed the device to rise slowly to the surface.
The research is part of a broader effort to monitor increasing levels of man-made noise in the world’s oceans — and their effect on marine life.
“Our goal is to make comprehensive sound maps of the ocean,” Dziak explained.
The team targeted the Challenger Deep for baseline recordings because they expected the slot-shaped canyon to be largely insulated from the global cacophony.
So they were surprised as they began analyzing the recordings.
The Mariana Trench is a seismically active subduction zone, where one tectonic plate dives under another, so earthquakes were a regular occurrence. Captured by the hydrophone, the rumble of the quakes builds in intensity like an approaching train.
Ship noise is higher-pitched and more rhythmic. The nearby island of Guam is on a major transoceanic route, so vessel traffic was a nearly constant source of noise in the environment, the researchers found.
The level of conversation between marine mammals picked up by the hydrophones was unexpected, Dziak said. Few whales dive much deeper than about 1.5 kilometres — yet their voices travelled clearly to the bottom of the trench.
A baleen whale of unknown type captured on the recordings seems to mimic a fog horn, while the deep-pitched calls of a toothed whale or dolphin sound like muffled thunder.
The hydrophone also recorded the roar of a typhoon, churning the surface about 300 kilometres north of the trench. “There was a huge amount of energy, high winds, big waves, that made the whole basin noisy,” Dziak said.
The project clearly shows that man-made noise is present even in some of the most remote corners of the planet. Next, the researchers plan to deploy hydrophones under the Arctic ice cap, to measure sound levels there before melting opens the area to more ship traffic.
And they hope to return to the Challenger Deep in 2017, this time deploying a camera along with the hydrophones.
