Using underwater microphones, called hydrophones, marine scientists several years ago picked up a puzzling, faint humming sound a thousand feet deep in the Pacific Ocean.

For years, no one could explain it. It didn't match the typical mating call of male humpback whales, nor the clicking of dolphins and other underwater creatures. And it appeared on a predictable schedule: for a couple of hours after sunset, and then for a couple more at dawn.

It was “more as if you're sitting on an airplane and it's humming, buzzing,” Simone Baumann-Pickering, a marine biologist at the Scripps Institution of Oceanography at the University of California in San Diego described to National Public Radio (NPR).

This week Dr. Baumann-Pickering and her team announced that they have linked the strange sound with the migration of billions of fish, crustaceans and squid from the dark depths of the Pacific – where they spend their days eluding predators – to the surface at night to feed. Baumann-Pickering estimates that the combined weight of the migrating fish adds up to 10 billion tons, possibly the largest migration of vertebrate animals on the planet, as NPR reports.

What actually produces the sound is not yet clear, though Baumann-Pickering suspects that it could be that the fish are humming or buzzing to communicate travel plans. Or maybe, she says, they are just passing gas.

"It's known that some fish are considered to be farting," Baumann-Pickering told NPR, "that they emit gas as they change depths in the water column." Fish have gas in their bladders to control their buoyancy.

Baumann-Pickering and other researchers will continue to study the largely unexplored cacophony of sounds resonating through our oceans to better understand how deep-sea creatures live and communicate.

"It’s a part of the world we know little about," David Gallo, an oceanographer at Woods Hole Oceanographic Institution in Massachusetts told Weather.com. He said Baumann-Pickering’s latest findings are among "the most fascinating research to come along in some time."

Underwater sounds are a particularly important area of study, as scientists have learned in recent years that too much man-made noise in the ocean – from ship traffic, oil and gas exploration, scientific research, and military sonar – can have harmful effects on its inhabitants.

Science has already shown that loud human activity in the ocean damages the hearing of whales, sometimes with fatal consequences. It interferes with their survival tactics.

The cries and clicks of whales are critical to their survival. They use these sounds to communicate with each other when looking for food, or trying to travel safely along uneven coastlines, or to and from breeding grounds. Some make loud noises to drive away prey.

"We are now starting to recognize chronic ocean noise as a ubiquitous habitat-level stressor,” Rob Williams, a marine conservation biologist for Seattle-based Oceans Initiative told Weather.com.

“If human-generated noise is masking this hum or buzz, we may be tipping the balance between predator and prey, and changing the way that ecosystems function,” Dr. Williams said.

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Scientific research of ocean sounds and their impact on marine life already has led to international guidelines aimed at hushing ship traffic.

In September, the US Navy said it will use sonar and other explosive training exercises more responsibly off the coasts of California and Hawaii to avoid harming dolphins, whales, and other marine mammals.