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Listening to the deep

Published on February 8th, 2015

by Tara Molle, US Coast Guard Petty Officer 2nd Class
We know more about the surface of the moon and Mars than we do about the ocean’s seafloor. With water encompassing 63.78 million square miles, the oceans cover 70 percent of the Earth’s surface, with the world’s largest body of water, the Pacific Ocean, covering roughly one third.

The Pacific also boasts the deepest trenches, specifically Challenger Deep in the Mariana Trench, which lies about 204 miles southwest of the U.S. territory of Guam and is nearly 7 miles deep. Given Challenger Deep’s inhospitable environment, no one has attempted to extensively record ambient sound at its full depth. That is, until now.

Housed in a cylindrical titanium pressure case, National Oceanic and Atmospheric Administration scientists designed a hydrophone capable of withstanding the 16,142 pounds of pressure per square inch in Challenger Deep. The ceramic pressure sensor is specifically designed for deep-ocean work, and houses the electronics, disk storage, pre-amplifier and battery case for a special mission.

Engineers at NOAA’s Pacific Marine Environmental Laboratory partnered with the Coast Guard to deploy the hydrophone into Challenger Deep using the Coast Guard’s unique capability of navigating to an exact position.

“Our plan is to make a baseline record of sound levels at Challenger Deep to investigate the levels of anthropogenic sound at these depths as well as gauge the contribution from natural sources such as submarine earthquakes and volcanoes,” said Dr. Robert Dziak, a research oceanographer with NOAA and a chief scientist of the project. “Hopefully we’ll also record sounds generated by unique animal life that exist at these depths.”

NOAA utilized the Coast Guard Cutter Sequoia, a 225-foot Juniper-class buoy tender home-ported in Apra Harbor, Guam, as a platform to deploy the hydrophone on Jan. 17, 2015.

“The joint mission was a great success and we are pleased to have been able to assist NOAA personnel in the deployment of their scientific equipment in such a remote location,” said Lt. Cmdr. Jessica Worst. “The operation was a perfect example of our 225-foot bouy tender’s versatility and multi-mission capability. While our positioning system and crane are normally used for servicing aids to navigation closer to the coast, the abilities of our equipment and crew easily translated over to this deep ocean scientific deployment.”

The ocean is full of ambient noise made by various things. Currents, animals and man-made noise from ship traffic generate sound waves in the deep blue sea, Dziak noted. He added that as with any scientific research, there are a lot of unknowns.

“The Challenger Deep should be one of the quietest places in the world’s oceans, with very little contribution from man-made noise, but we won’t know for sure until we make these measurements,” said Dziak. “This project also represents true exploration and we don’t know exactly what we’ll record. We don’t fully know what physical or biological processing are occurring down at these depths or what type of sounds the animal life there may be producing.”

Man-made noise heavily impacts the ocean and has also increased over the years. The hydrophone will be used to listen and see if man-made noise will make it into the trench’s seemingly endless depth. In fact, if you placed Mount Everest at the trench’s seafloor, you’d still have over a mile of water left above its peak.

The daunting task at hand required the hydrophone to free-fall approximately six hours one-way to the seafloor. The hydrophone cylinders are attached to several miles of mooring line and floats. It will be left at the seabed for three to four weeks to capture ambient sound needed to conduct the research. Once it’s retrieved, NOAA will release the raw, unprocessed sound for public download and release short recordings of interesting sounds on their website to highlight the project.

“We are cautiously optimistic that recovery will go smoothly and that the hydrophone will have recorded data,” said Dziak. “If we successfully record sound data and become one of the first, if not the first to make recordings at Challenger Deep, then our excitement will be off the charts!”

We are now left to our own imaginations to the sights and now sounds of what lies within Challenger Deep. What could we hear? Will the thousands of life forms inhabiting the oceans and commercial ships transiting the seas daily produce sounds to that depth? Is the Megalodon lurking below waiting to be rediscovered, or is the Challenger Deep truly the quietest place on Earth? Only time will tell.

For more information, visit the NOAA PMEL Acoustics site at:


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