Researchers have for the first time eavesdropped on the deepest part of the Pacific Ocean. Surprisingly, they found a cacophony of sounds - natural and man-made.
Researchers from the National Oceanic and Atmospheric Administration (NOAA), Oregon State University and the U.S. Coast Guard sent a titanium-encased hydrophone, built to withstand extreme pressure, more than seven miles below the ocean's surface, to an area known as Challenger Deep trough.
This trough, the deepest known point of any ocean on the planet, lies at the southern end of the Mariana Trench near the tiny island nation of Micronesia. While scientists expected to hear silence, they actually found the deep ocean was quite noisy.
"You would think that the deepest part of the ocean would be one of the quietest places on Earth," said Robert Dziak, a NOAA research oceanographer and chief scientist on the project, in a news release. "Yet there really is almost constant noise from both natural and man-made sources. The ambient sound field at Challenger Deep is dominated by the sound of earthquakes, both near and far was well as the distinct moans of baleen whales and the overwhelming clamor of a category 4 typhoon that just happened to pass overhead."
The hydrophone also picked up on whirring ship propellers, likely caused by container ships traveling through nearby Guam, which is a freight-shipping hub in the area.
Within the past several decades, man-made noise has greatly increased underwater. It is believed that this noise "pollution" interferes with the navigating, feeding and breeding patterns of marine life, such as fish and whales.
The goal of the study was to identify a baseline for ambient noise in the deepest part of the Pacific Ocean, so that researchers can better asses how noise levels are growing in the future.
"We recorded a loud magnitude 5.0 earthquake that took place at a depth of about 10 kilometers (or more than six miles) in the nearby ocean crust," Dziak explained. "Since our hydrophone was at 11 kilometers, it actually was below the earthquake, which is really an unusual experience. The sound of the typhoon was also dramatic, although the cacophony from big storms tends to be spread out and elevates the overall noise for a period of days."
"We had never put a hydrophone deeper than a mile or so below the surface," said Oregon State ocean engineer Haru Matsumoto, "so putting an instrument down some seven miles into the ocean was daunting."
