The ocean is a noisy place, even at its deepest spot.
That came as a surprise to a team of researchers who sent a titanium-encased recorder nearly seven miles below the surface to eavesdrop on the ocean floor.
“You would think that the deepest part of the ocean would be one of the quietest places on Earth,” National Oceanic and Atmospheric Administration research oceanographer Robert Dziak, said in a statement. “Yet there really is almost constant noise from both natural and man-made sources.”
Dziak was the chief scientist for the project whose mission was to obtain a baseline recording — from Challenger Deep, the lowest point in the Mariana Trench — for future scientists to use in measuring the change in underwater noise levels. The team was composed of researchers from NOAA, Oregon State University and the U.S. Coast Guard and was funded by NOAA.
They found a cacophony dominated by earthquakes, as well as the moans of baleen whales and the sound of a typhoon passing above.
“There was also a lot of noise from ship traffic, identifiable by the clear sound pattern the ship propellers make when they pass by,” Dziak said. Guam, a regional shipping hub, is very close to Challenger Deep.
Recovering audio from the ocean’s deepest point required overcoming pressure more than 1,000 times greater than that at the surface. (Atmospheric pressure on land is roughly 15 pounds per square inch. At the bottom of the Mariana Trench, it is greater than 16,000 PSI.)
“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 Oregon State ocean engineer Haru Matsumoto, who helped design the hydrophone used to make the recording.
The device was in free-fall for six hours before it hit the ocean floor last July. It reached its storage capacity for audio recording over the course of about 23 days, but remained on the floor until November when conditions were right for it to float back to the surface — a process triggered by an acoustic signal sent from above.
Over the course of several months, the team began piecing together what they recorded: a magnitude 5.0 earthquake, centered in the ocean’s crust nearly a mile above the device; a typhoon; big storms that drove up the ambient noise for a period of days; and even waves and winds at the surface.
“Sound doesn’t get as weak as you think it does, even that far from the source,” Matsumoto said.
The researchers plan to conduct a similar project next year, for a longer period and with a camera attached.
Listen to five recordings from last year’s deployment below: