Researchers used waterproof microphones to eavesdrop on the deepest parts of the Pacific Ocean and announced Wednesday that they were surprised to hear a cacophony of noise-natural and man-made.

For three weeks, a team of researchers from the National Oceanic and Atmospheric Administration (NOAA), Oregon State University and the U.S. Coast Guard used a waterproof microphone to listen to the ocean 11 kilometers (7 miles) under the surface. According to Anadolu Agency, The research was conducted in an area known as Challenger Deep trough, the deepest known point of any ocean on the planet.

The trough is 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 quite noisy.

“You would think that the deepest part of the ocean would be one of the quietest places on Earth,” Robert Dziak, a NOAA oceanographer and chief project scientist, said in a statement. “Yet 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 clamor of a category 4 typhoon that just happened to pass overhead.”

Scientists also heard the sound of whirring ship propellers, likely caused by container ships traveling through nearby Guam, a freight shipping hub in the area.

Researchers wanted to listen to the Challenger Deep in order to find a baseline for ambient noise in the deep ocean, which they believed humans are making it much noisier.

The team believes increasing noise due to human activity could interfere with the navigating, feeding and breeding patterns of marine life including fish and whales.

In order to listen in on the tough, which is deep enough to hold Mount Everest, scientists had to develop a special, titanium-encased hydrophone built to withstand extreme pressure.

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