The baleen whale let loose a deep, eerie honk. The earthquake rumbled out of nowhere like a coming train. But the most startling sound recorded deep in the ocean just droned.

It came from a boat propeller on the surface, passing seven miles above.

Robert Dziak, the National Oceanic and Atmospheric Administration research oceanographer who led the recent Challenger Deep project dropping a hydrophone into the depths off Guam, knew there would be shipping traffic near the island, a container ship hub. But seven miles down, he expected to find one of the quietest places on Earth.

“It was a little unexpected to detect ship propellers with such clarity, at such a great depth,” he said. The deep bottom turned out to be a virtual jungle of sounds — the whale calls and quake rumble, the tumult of a typhoon passing overhead. The noise was continual, he said.

The depth reached by the boat propeller drone was no real surprise to conservationists who have argued for years that human noise in the ocean is more pervasive and damaging than many realize.

“You go into a pool, make a noise and it sounds muffled, but the opposite is true. Noise travels quicker and farther in the water than in air,” said Zak Smith, Natural Resources Defense Council senior attorney in the marine mammal division.

Offshore the United States, the Atlantic Ocean is now considered urban with heavy ship traffic, open to high-decibel military sonar training and under consideration to be leased to seismic testing for oil, natural gas and minerals — loud blasts that sound every 16 seconds or so.

Permitting these activities requires mitigations based on sound zones, areas around the noise maker where the sound would be so loud to permanently injure a marine animal.

“That’s always where the debate is,” Smith said. More importantly, and what gets lost, is that studies are beginning to show constant or repeated noise below those levels can disrupt behavior such as foraging or reproducing, he said. And that can threaten species survival.

Findings such as the boat propeller noise “can help change people’s understanding of that. It shows the unintended consequences of our activities,” Smith said.

Asked if the propeller-noise finding in the project could lead to a reassessment of “safe” levels of noise in permitted undersea activities, Dziak said, “We are really now just at the stage of trying to measure baseline sound levels at various ocean areas, and trying to understand ocean noise impacts on marine ecosystems.”

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