WEBVTT - Is the Earth Humming to Us?

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<v Speaker 1>Welcome to brain Stuff from How Stuff Works. Hey, brain Stuff,

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<v Speaker 1>Lauren vogelbam here. Think of a parked truck with the

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<v Speaker 1>engine running. That's the sound some people have compared to

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<v Speaker 1>the mysterious NonStop noise which is emanating from this our

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<v Speaker 1>home planet. We've been aware of this phenomenon for decades now,

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<v Speaker 1>and while the source of the commotion remains unknown, the

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<v Speaker 1>scientists who study it have made an important breakthrough. They

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<v Speaker 1>finally recorded it. In the nineteenth century, geologists began to

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<v Speaker 1>suspect that the Earth might be producing a constant hum,

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<v Speaker 1>one which rings out even in the absence of earthquakes

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<v Speaker 1>and seismic events. They also reasoned that the noise must

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<v Speaker 1>be too quiet for our human ear drums to hear.

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<v Speaker 1>The official name for this drone is permanent free oscillations.

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<v Speaker 1>Until somewhat recently, its existence was only theoretical. A team

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<v Speaker 1>led by seismologist Hugo Benioff did try to detect the

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<v Speaker 1>signal in ninety nine, but their efforts failed because the

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<v Speaker 1>time science did not yet possess any instruments that were

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<v Speaker 1>sensitive enough to pick up the hum. Theory became fact

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<v Speaker 1>with the advance of technology in scientists at the Showa Station,

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<v Speaker 1>a Japanese research base in eastern Antarctica. We're finally able

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<v Speaker 1>to prove that permanent free oscillations really do exist. The

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<v Speaker 1>good news was announced a year later when the show

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<v Speaker 1>A team published their findings. Since then, numerous other teams

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<v Speaker 1>have observed the same noise. Now, for the first time ever,

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<v Speaker 1>the Earth's ham has been recorded using seizeman equipment on

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<v Speaker 1>the ocean floor. This is a big deal because every

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<v Speaker 1>previous study which has documented the noise did so with

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<v Speaker 1>land based instruments. The achievement was a hard won prize

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<v Speaker 1>for Martha Dean and her team. She's a geophysicist with

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<v Speaker 1>the Paris Institute of Earth Physics. Under her leadership, the

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<v Speaker 1>international team reviewed data collected over an eleven month period

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<v Speaker 1>from fifty seven sizemometer stations on the floor of the

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<v Speaker 1>Indian Ocean, and that was just the first step. Next,

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<v Speaker 1>the researchers eliminated all forms of audio interference such as

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<v Speaker 1>water currents and technical glitches from the recordings made at

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<v Speaker 1>two of the stations. With the deletion of this extra noise,

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<v Speaker 1>Dean and her colleagues could finally isolate the hum. They

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<v Speaker 1>were looking for. Why was it so important to record

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<v Speaker 1>the oscillations with submerged seismometers. As Dean told us in

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<v Speaker 1>an email, these instruments will broaden our perspective in a

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<v Speaker 1>way that terrestrial tools never could. She said, ocean bottom

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<v Speaker 1>sizedmometers can cover much larger areas than land based ones.

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<v Speaker 1>For the ocean covers seventy of our planet. She added

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<v Speaker 1>that we can better understand the phenomena by studying the

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<v Speaker 1>HUM signal at places far from land or islands. Maybe

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<v Speaker 1>one day we'll even be able to pinpoint its source.

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<v Speaker 1>You see, nobody knows exactly how the HUM is being made.

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<v Speaker 1>A few different hypotheses have been put forth. Some geophysicists

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<v Speaker 1>think gets generated by the ceaseless pounding of ocean waves

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<v Speaker 1>onto continental slopes. Others believe that it could be the

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<v Speaker 1>product of atmospheric turbulence and global wind patterns. But if

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<v Speaker 1>that second explanation is true, we'd expect the rumblings amplitude,

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<v Speaker 1>that is, its loudness, to vary from season to season.

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<v Speaker 1>Previous studies have claimed that this is happening, yet the

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<v Speaker 1>new research says otherwise. Dean's group confirmed that the hums

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<v Speaker 1>pitch rises and falls, with its maximum volume hitting a

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<v Speaker 1>frequency of four point five milliherts. That's about ten thousand

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<v Speaker 1>times softer than the faintest noises our ears can detect. However,

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<v Speaker 1>according to the team's findings that amplitude changes don't correlate

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<v Speaker 1>with seasonality. Thus, Dean and her colleagues argue that atmospheric

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<v Speaker 1>issues alone cannot account for the existence of these permanent

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<v Speaker 1>free oscillations. They also think that their research could open

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<v Speaker 1>the door for future research on the Earth's interior. Geologists

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<v Speaker 1>use a process called tomography to map out the inside

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<v Speaker 1>of our world. Think of it as a large scale

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<v Speaker 1>m r I scan. Dean explains that scientists invert the

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<v Speaker 1>recordings of seismic waves to decipher the makeup of various

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<v Speaker 1>layers and structures within the planet. Going forward, ocean bottom

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<v Speaker 1>sizedmometers like those used in her recent study should give

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<v Speaker 1>tomographers more data to work with. Hopefully we'll soon have

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<v Speaker 1>a better idea of what lies beneath our feet. Today's

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<v Speaker 1>episode was written by Mark Bancini and produced by Tyler Clang.

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<v Speaker 1>For more in this and lots of other earth shaking topics,

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<v Speaker 1>visit our home Planet, how Stuff Works dot Com.