CHALLENGER DEEP – Scientists have recorded a cacophony of sounds including ship propellers at the deepest part of the world’s oceans, more than 10,000 metres below the surface.

For three weeks, researchers used an underwater microphone, known as a hydrophone, to record ambient noise in the Challenger Deep trough in the Mariana Trench near Micronesia.

“You would think that the deepest part of the ocean would be one of the quietest places on Earth,” National Oceanic and Atmospheric Administration (NOAA) research oceanographer Robert Dziak said in a statement.

“Yet there really is almost constant noise from both natural and man-made sources.”

The NOAA conducted the research along with Oregon State University and the US Coast Guard.

The recordings revealed the sounds of earthquakes, the moans of baleen whales and clamour from a category four typhoon.

“There was also a lot of noise from ship traffic, identifiable by the clear sound pattern the ship propellers make when they pass by,” Professor Dziak said.

Challenger Deep is close to Guam, a regional hub for container shipping with China and the Philippines.

The research was funded by the NOAA to establish a baseline for ambient noise so that scientists in the future will be able to determine if noise caused by human activity is growing.

Engineers develop special deep water hydrophone

The bottom of Challenger Deep has been measured at between 10,916 and 10,994 metres deep, making it about two kilometres deeper than Mount Everest is tall.

Pressure at the bottom is about 16,000 pounds per square inch (psi), approximately 1099 times greater than air pressure at sea level.

“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,” said engineer Haru Matsumoto, who helped develop a titanium-cased hydrophone capable of withstanding such pressure.

“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.”

It took more than six hours for the hydrophone to freefall to Challenger Deep after being deployed in July 2015.

The hydrophone’s on-board flash drive filled with sound data in about 23 days, but shipping schedules and persistent typhoons meant researchers had to wait until November to retrieve it.

Researchers used an acoustic signal to trigger the hydrophone’s release, and it ascended using attached floats.

“It is akin to sending a deep-space probe to the outer solar system. We’re sending out a deep-ocean probe to the unknown reaches of inner space,” Professor Dziak said.

Since retrieving the hydrophone, the researchers spent months analysing the recordings and sorting natural sounds from ships and other human activities.

“We recorded a loud magnitude 5.0 earthquake that took place at a depth of about 10 kilometres in the nearby ocean crust,” Professor Dziak said.

“Since our hydrophone was at 11 kilometres, it actually was below the earthquake, which is really an unusual experience.”

The typhoon that passed overhead increased overall noise for several days.

Matsumoto said waves and winds disturbing the sea surface could also be detected.

“Sound doesn’t get as weak as you think it does even that far from the source,” he said.

A return to Challenger Deep is planned for 2017, including a deep-ocean camera and a longer deployment of the hydrophone.

- ABC