Scientists have eavesdropped on the deepest part of the world's oceans and instead of finding a sea of silence, they discovered a cacophony of sounds both natural and caused by humans.
For three weeks, a titanium-encased hydrophone recorded ambient noise from the ocean floor at a depth of more than 36,000 feet in a trough known as Challenger Deep in the fabled Mariana Trench near Micronesia.
"You would think that the deepest part of the ocean would be one of the quietest places on Earth," said Robert Dziak, a National Oceanic and Atmospheric Administration (NOAA) research oceanographer and chief scientist on the project.
"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 clamour of a category 4 typhoon that just happened to pass overhead," he said.
"There was also a lot of noise from ship traffic, identifiable by the clear sound pattern the ship propellers make when they pass by," added Dziak.
The project was designed to establish a baseline for ambient noise in the deepest part of the Pacific Ocean.
Human-caused noise has increased steadily over the past several decades and getting these first recordings will allow scientists in the future to determine if the noise levels are growing.
The bottom of the Challenger Deep trough is roughly 11 km below the ocean's surface. In fact, you could put the world's tallest peak - Mount Everest - in the trench and its top would still be more than a mile from the surface.
The pressure at that depth is incredible, said Haru Matsumoto, an Oregon State University ocean engineer who along with NOAA engineer Chris Meinig helped to develop a hydrophone capable of withstanding such pressure.
In the average person's home or office, the atmospheric pressure is about 14.7 pounds per square inch; at the bottom of the Mariana Trench, it is more than 16,000 PSI.
Partnering with the US Coast Guard, the researchers deployed the hydrophone from the Guam-based cutter Sequoia in July, last year.
For the past several months, Dziak and his colleagues have been analysing the sounds and differentiating 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," Dziak said.
"Since our hydrophone was at 11 kilometres, 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," he said.
Matsumoto said the hydrophone also picked up a lot of noise from the surface of the ocean - some 11 km above - including waves and winds disturbing the surface.
