Scientists have recorded sounds from the deepest part of the ocean — and they were amazed by what they heard.

Researchers used a titanium-encased microphone, or hydrophone, to detect sound waves more than 36,000 feet beneath the surface of the Pacific Ocean, in the Challenger Deep seabed at the bottom of the Mariana Trench near Guam.

The National Oceanic and Atmospheric Administration, Oregon State University and the U.S. Coast Guard carried out the project, which involved recording sound over 23 days.

While you might expect the deepest part of the ocean to be among the quietest places on Earth, the recordings captured almost constant noise from both natural and man-made sources, said Robert Dziak, an oceanographer at NOAA.

“To my knowledge, there’s never been a sound recording made at Challenger Deep — certainly not one as long as ours,” he told The Huffington Post.

The Bottom Of The Ocean Is Amazingly Noisy 03 MarchThe recordings, which can be heard below, are dominated by noise from earthquakes, but they also capture engine sounds from passing ships and the moans of baleen whales.

The trench is so noisy because its acoustic properties allow sound from the ocean’s surface to travel far below, Dziak explained. “Sounds at the sea surface — ship traffic and whale calls — propagate down very effectively into the trench,” he said.

And the pressure at the bottom is so intense — 16,000 pounds per square inch — that researchers had to sink the hydrophone gradually over six hours to avoid cracking it. They captured the recordings in July, but weren’t able to retrieve the hydrophone until November, due to storms and ocean traffic.

While the recordings may sound muddy to human ears, they reveal valuable information about what’s happening in the ocean and enable scientists to establish a baseline level of sound at the ocean’s deepest point. Having those measurements makes it possible to track sound levels in the ocean over time, according to Dziak.

The new research could help scientists better understand how man-made noise affects marine animals. “We really don’t know how sound is affecting marine ecosystems,” he said.

Researchers may also use a hydrophone to record sounds in other parts of the ocean. “I’m very interested in getting this under the ice cap in the Arctic Ocean,” Dziak added.

The scientists hope to return to Challenger Deep in 2017 to deploy the hydrophone for a longer period of time and also attach a deep-ocean camera.

“It is akin to sending a deep-space probe to the outer solar system,” Dziak said of the research in a statement. “We’re sending out a deep-ocean probe to the unknown reaches of inner space.”

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