Speaker 1: Welcome to brain Stuff from How Stuff Works. Hey, brain Stuff,
Speaker 1: Lauren vogelbam here. Think of a parked truck with the
Speaker 1: engine running. That's the sound some people have compared to
Speaker 1: the mysterious NonStop noise which is emanating from this our
Speaker 1: home planet. We've been aware of this phenomenon for decades now,
Speaker 1: and while the source of the commotion remains unknown, the
Speaker 1: scientists who study it have made an important breakthrough. They
Speaker 1: finally recorded it. In the nineteenth century, geologists began to
Speaker 1: suspect that the Earth might be producing a constant hum,
Speaker 1: one which rings out even in the absence of earthquakes
Speaker 1: and seismic events. They also reasoned that the noise must
Speaker 1: be too quiet for our human ear drums to hear.
Speaker 1: The official name for this drone is permanent free oscillations.
Speaker 1: Until somewhat recently, its existence was only theoretical. A team
Speaker 1: led by seismologist Hugo Benioff did try to detect the
Speaker 1: signal in ninety nine, but their efforts failed because the
Speaker 1: time science did not yet possess any instruments that were
Speaker 1: sensitive enough to pick up the hum. Theory became fact
Speaker 1: with the advance of technology in scientists at the Showa Station,
Speaker 1: a Japanese research base in eastern Antarctica. We're finally able
Speaker 1: to prove that permanent free oscillations really do exist. The
Speaker 1: good news was announced a year later when the show
Speaker 1: A team published their findings. Since then, numerous other teams
Speaker 1: have observed the same noise. Now, for the first time ever,
Speaker 1: the Earth's ham has been recorded using seizeman equipment on
Speaker 1: the ocean floor. This is a big deal because every
Speaker 1: previous study which has documented the noise did so with
Speaker 1: land based instruments. The achievement was a hard won prize
Speaker 1: for Martha Dean and her team. She's a geophysicist with
Speaker 1: the Paris Institute of Earth Physics. Under her leadership, the
Speaker 1: international team reviewed data collected over an eleven month period
Speaker 1: from fifty seven sizemometer stations on the floor of the
Speaker 1: Indian Ocean, and that was just the first step. Next,
Speaker 1: the researchers eliminated all forms of audio interference such as
Speaker 1: water currents and technical glitches from the recordings made at
Speaker 1: two of the stations. With the deletion of this extra noise,
Speaker 1: Dean and her colleagues could finally isolate the hum. They
Speaker 1: were looking for. Why was it so important to record
Speaker 1: the oscillations with submerged seismometers. As Dean told us in
Speaker 1: an email, these instruments will broaden our perspective in a
Speaker 1: way that terrestrial tools never could. She said, ocean bottom
Speaker 1: sizedmometers can cover much larger areas than land based ones.
Speaker 1: For the ocean covers seventy of our planet. She added
Speaker 1: that we can better understand the phenomena by studying the
Speaker 1: HUM signal at places far from land or islands. Maybe
Speaker 1: one day we'll even be able to pinpoint its source.
Speaker 1: You see, nobody knows exactly how the HUM is being made.
Speaker 1: A few different hypotheses have been put forth. Some geophysicists
Speaker 1: think gets generated by the ceaseless pounding of ocean waves
Speaker 1: onto continental slopes. Others believe that it could be the
Speaker 1: product of atmospheric turbulence and global wind patterns. But if
Speaker 1: that second explanation is true, we'd expect the rumblings amplitude,
Speaker 1: that is, its loudness, to vary from season to season.
Speaker 1: Previous studies have claimed that this is happening, yet the
Speaker 1: new research says otherwise. Dean's group confirmed that the hums
Speaker 1: pitch rises and falls, with its maximum volume hitting a
Speaker 1: frequency of four point five milliherts. That's about ten thousand
Speaker 1: times softer than the faintest noises our ears can detect. However,
Speaker 1: according to the team's findings that amplitude changes don't correlate
Speaker 1: with seasonality. Thus, Dean and her colleagues argue that atmospheric
Speaker 1: issues alone cannot account for the existence of these permanent
Speaker 1: free oscillations. They also think that their research could open
Speaker 1: the door for future research on the Earth's interior. Geologists
Speaker 1: use a process called tomography to map out the inside
Speaker 1: of our world. Think of it as a large scale
Speaker 1: m r I scan. Dean explains that scientists invert the
Speaker 1: recordings of seismic waves to decipher the makeup of various
Speaker 1: layers and structures within the planet. Going forward, ocean bottom
Speaker 1: sizedmometers like those used in her recent study should give
Speaker 1: tomographers more data to work with. Hopefully we'll soon have
Speaker 1: a better idea of what lies beneath our feet. Today's
Speaker 1: episode was written by Mark Bancini and produced by Tyler Clang.
Speaker 1: For more in this and lots of other earth shaking topics,
Speaker 1: visit our home Planet, how Stuff Works dot Com.