Sounds of the Deep: First recordings from the deepest point on Earth

From moans of whales to the put-put-put of ships’ propellers, from the rumble of earthquakes to the roar of typhoons: scientists release first recordings from the deepest point on Earth.

By: Agencies | Updated: March 22, 2016 9:40 am

Last July, an underwater microphone was lowered into the Challenger Deep, the deepest point in the Mariana Trench, the 2,550-km-long and 70-km-wide trough on the Pacific Ocean floor to the east of the Philippines, the deepest part of the world’s oceans. The scientists, from the US National Oceanic and Atmospheric Administration, Oregon State University and US Coast Guard, had expected a deep silence; instead, they heard a cacophony of sounds, both natural and created by humans.

The NOAA-funded project was intended to establish a baseline for ambient noise in the Pacific’s deepest part. Human-created noise has increased steadily in recent decades, and scientists in the future need data to determine how this might affect marine animals that use sound to communicate, navigate and feed — whales, dolphins and fish. The hydrophone stayed underwater for about 3 weeks; the next mission, in early 2017, will deploy it for longer, and also attach a deep-ocean camera.

The Site

Challenger Deep is at the southern tip of Mariana Trench, near Micronesia, close to the US island territory of Guam, the regional hub for US container shipping with China and Philippines.

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The Sounds

HOW did the sounds reach the hydrophone? Sound travels faster in water than in air; the distance it travels depends upon ocean temperature and pressure. Pressure keeps increasing with depth, but temperature stops falling after a point. The ripple-like sound waves from, say, the call of a whale, slow down as depth increases (and temperature falls), thus causing them to refract downward. Once the waves reach the bottom of the thermocline layer (600 ft-3,300 ft, corresponding to dysphotic and mesopelagic zones; right) their speed reaches its minimum. Below the thermocline, temperature is constant, but pressure continues to increase. This causes the speed of sound to increase and makes the waves refract upward. This “channeling” allows sound waves to travel thousands of miles without the signal losing too much energy. Placed at a proper depth, hydrophones can pick up whale songs and manmade noises like ship propellers many miles away.

The Hydrophone

Made of ceramic, especially cased in titanium to withstand the enormous under-ocean pressure. Was lowered and pulled up 36,000 ft, or about 11 km, at a speed no more than 5 m/second (or 18 km/hr) to ensure it was not crushed by rapid pressure changes.

The Hydrophone

Made of ceramic, especially cased in titanium to withstand the enormous under-ocean pressure. Was lowered and pulled up 36,000 ft, or about 11 km, at a speed no more than 5 m/second (or 18 km/hr) to ensure it was not crushed by rapid pressure changes.

The Exploration

The hydrophone stayed 23 days starting July 2015 at the bottom of the Deep, recording the almost constant sound from both far and near until its flash drive was full. It remained tethered to the bottom, however, until weather and a break in ocean traffic allowed the scientists to return to recover it in November. The results were reported in March. “The ambient sound field is dominated by the sound of earthquakes, both near and far, as well as moans of baleen whales, and the clamour of a category 4 typhoon that happened to pass overhead,” said Robert Dziak, the chief project scientist.

The Depth

The average depth of the world’s oceans is about 12,100 ft. The Challenger Deep is three times the average — approximately 36,200 ft deep. It is named after the HMS Challenger, whose crew first sounded the depths of the trench in 1875.

The Pressure

Pressure at sea level is 14.5 PSI (pounds/sq inch), something we don’t feel because fluids inside the body are pushing back with the same force. Diving to even some depth, however, results in increased pressure on the eardrums. Descending every 33 ft (10 m) under the surface increases the pressure by 14.5 PSI — which means the pressure in the deepest ocean is 16,000 PSI. This is roughly equivalent to the weight of an elephant acting on an area as big as a postage stamp — or of one person trying to hold up 50 jumbo jets. This pressure does not, however, crush whales, whose ribs are encased in loose, flexible cartilage, and whose lungs are able to collapse so as not to rupture with pressure changes.

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