Seven miles deep, oceans still a noisy place

Eavesdropping into the deepest part of the ocean, the US researchers discovered a cacophony of sounds both natural and caused by humans.

The researchers recorded for three weeks, the ambient noise from the ocean floor at a depth of more than 36,000 feet using a titanium-encased hydrophone, dropped down in the Challenger Deep trough -- the deepest known point in the Earth's seabed hydrosphere, in the Pacific Ocean.

"There is almost constant noise. The ambient sound field is dominated by the sound of earthquakes, both near and far, as well as distinct moans of baleen whales, and the clamour of a category 4 typhoon that just happened to pass overhead," said Robert Dziak, oceanographer and chief project scientist at National Oceanic and Atmospheric Administration (NOAA).

The hydrophone also picked up sound from ship propellers.

Arguing that the human-created noise has increased steadily in recent decades, the researchers noted that the recordings will help scientists to determine how this might affect marine animals that use sound to communicate, navigate and feed, such as whales, dolphins and fish.

Researchers deployed the hydrophone from the Guam-based US Coast Guard cutter Sequoia in July 2015. The device recorded sound continuously over 23 days, completely filling the flash drive.

However, scientists had to wait until November to retrieve the hydrophone due to ships' schedules and persistent typhoons. The device remained anchored to the seafloor until scientists returned.

The project, undertaken by from NOAA, Oregon State University, and the US Coast Guard was designed to establish a baseline for ambient noise in the deepest part of the Pacific Ocean.