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You might expect the middle of the Pacific Ocean to be a pretty quiet place, especially a thousand feet down, but huge parts of the ocean are humming. Scientists have been trying to figure out what it is for several years. And as NPR's Christopher Joyce reports, they now have their answer, and it's a real surprise.
CHRISTOPHER JOYCE, BYLINE: We usually hear the ocean when it meets the land where waves roll up onto the beach. But marine scientists like to listen to the deep ocean where they drop hydrophones overboard - underwater microphones - to listen for things like humpback whales calling to each other.
(SOUNDBITE OF WHALE CALL)
JOYCE: But a few years ago, those hydrophones picked up something weird. It was faint but different from the normal background sound.
SIMONE BAUMANN-PICKERING: It's definitely more a - as if you're sitting on an airplane, and it's humming, buzzing.
JOYCE: Simone Baumann-Pickering is a biologist at the Scripps Institute of Oceanography in California. She says the sound starts as the sun sets, stops overnight then picks up again for two hours at dawn. Now, biologists know that huge clouds of small fish hide in the dark, deep water during the day and then rise up near the surface to feed at night, and the hydrophones revealed that the fish make the ocean hum when the rise and fall. It could be they're signaling each other that it's time to move, or...
BAUMANN-PICKERING: It could be - for example, it's known that some fish are considered to be farting - that they emit gas as they change in the depths in the water column.
JOYCE: The gas comes from a swim bladder inside the fish that controls its buoyancy. Imagine billions of fish literally jetting up and down in the ocean every day. This would have to be the largest migration on the planet.
BAUMANN-PICKERING: Oh, for sure, absolutely.
JOYCE: Baumann-Pickering described her research at an ocean science meeting today. She says it raises an interesting question. Could all this noise be like a dinner bell attracting large predators to these clouds of small fish? Nobody knows, she says. Christopher Joyce, NPR News.
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