1 00:00:02,040 --> 00:00:07,120 Speaker 1: Welcome to brain Stuff from How Stuff Works. Hey, brain Stuff, 2 00:00:07,160 --> 00:00:10,480 Speaker 1: Lauren vogelbam here. Think of a parked truck with the 3 00:00:10,560 --> 00:00:14,040 Speaker 1: engine running. That's the sound some people have compared to 4 00:00:14,080 --> 00:00:17,680 Speaker 1: the mysterious NonStop noise which is emanating from this our 5 00:00:17,880 --> 00:00:21,439 Speaker 1: home planet. We've been aware of this phenomenon for decades now, 6 00:00:21,680 --> 00:00:24,560 Speaker 1: and while the source of the commotion remains unknown, the 7 00:00:24,600 --> 00:00:27,840 Speaker 1: scientists who study it have made an important breakthrough. They 8 00:00:27,960 --> 00:00:32,440 Speaker 1: finally recorded it. In the nineteenth century, geologists began to 9 00:00:32,479 --> 00:00:35,680 Speaker 1: suspect that the Earth might be producing a constant hum, 10 00:00:35,840 --> 00:00:38,520 Speaker 1: one which rings out even in the absence of earthquakes 11 00:00:38,520 --> 00:00:41,920 Speaker 1: and seismic events. They also reasoned that the noise must 12 00:00:41,960 --> 00:00:44,480 Speaker 1: be too quiet for our human ear drums to hear. 13 00:00:45,200 --> 00:00:48,560 Speaker 1: The official name for this drone is permanent free oscillations. 14 00:00:49,200 --> 00:00:53,000 Speaker 1: Until somewhat recently, its existence was only theoretical. A team 15 00:00:53,080 --> 00:00:56,240 Speaker 1: led by seismologist Hugo Benioff did try to detect the 16 00:00:56,280 --> 00:01:00,160 Speaker 1: signal in ninety nine, but their efforts failed because the 17 00:01:00,200 --> 00:01:03,000 Speaker 1: time science did not yet possess any instruments that were 18 00:01:03,000 --> 00:01:06,839 Speaker 1: sensitive enough to pick up the hum. Theory became fact 19 00:01:06,920 --> 00:01:11,720 Speaker 1: with the advance of technology in scientists at the Showa Station, 20 00:01:11,840 --> 00:01:15,479 Speaker 1: a Japanese research base in eastern Antarctica. We're finally able 21 00:01:15,520 --> 00:01:19,319 Speaker 1: to prove that permanent free oscillations really do exist. The 22 00:01:19,360 --> 00:01:21,440 Speaker 1: good news was announced a year later when the show 23 00:01:21,520 --> 00:01:25,039 Speaker 1: A team published their findings. Since then, numerous other teams 24 00:01:25,040 --> 00:01:28,280 Speaker 1: have observed the same noise. Now, for the first time ever, 25 00:01:28,520 --> 00:01:31,520 Speaker 1: the Earth's ham has been recorded using seizeman equipment on 26 00:01:31,560 --> 00:01:34,560 Speaker 1: the ocean floor. This is a big deal because every 27 00:01:34,560 --> 00:01:37,160 Speaker 1: previous study which has documented the noise did so with 28 00:01:37,319 --> 00:01:41,200 Speaker 1: land based instruments. The achievement was a hard won prize 29 00:01:41,240 --> 00:01:44,360 Speaker 1: for Martha Dean and her team. She's a geophysicist with 30 00:01:44,400 --> 00:01:47,760 Speaker 1: the Paris Institute of Earth Physics. Under her leadership, the 31 00:01:47,800 --> 00:01:51,200 Speaker 1: international team reviewed data collected over an eleven month period 32 00:01:51,320 --> 00:01:54,200 Speaker 1: from fifty seven sizemometer stations on the floor of the 33 00:01:54,200 --> 00:01:57,840 Speaker 1: Indian Ocean, and that was just the first step. Next, 34 00:01:57,920 --> 00:02:01,360 Speaker 1: the researchers eliminated all forms of audio interference such as 35 00:02:01,400 --> 00:02:04,640 Speaker 1: water currents and technical glitches from the recordings made at 36 00:02:04,680 --> 00:02:08,160 Speaker 1: two of the stations. With the deletion of this extra noise, 37 00:02:08,280 --> 00:02:11,160 Speaker 1: Dean and her colleagues could finally isolate the hum. They 38 00:02:11,160 --> 00:02:14,600 Speaker 1: were looking for. Why was it so important to record 39 00:02:14,639 --> 00:02:18,400 Speaker 1: the oscillations with submerged seismometers. As Dean told us in 40 00:02:18,400 --> 00:02:21,320 Speaker 1: an email, these instruments will broaden our perspective in a 41 00:02:21,360 --> 00:02:25,360 Speaker 1: way that terrestrial tools never could. She said, ocean bottom 42 00:02:25,400 --> 00:02:28,880 Speaker 1: sizedmometers can cover much larger areas than land based ones. 43 00:02:29,120 --> 00:02:32,760 Speaker 1: For the ocean covers seventy of our planet. She added 44 00:02:32,760 --> 00:02:35,520 Speaker 1: that we can better understand the phenomena by studying the 45 00:02:35,560 --> 00:02:40,160 Speaker 1: HUM signal at places far from land or islands. Maybe 46 00:02:40,160 --> 00:02:42,360 Speaker 1: one day we'll even be able to pinpoint its source. 47 00:02:43,000 --> 00:02:45,960 Speaker 1: You see, nobody knows exactly how the HUM is being made. 48 00:02:46,440 --> 00:02:50,240 Speaker 1: A few different hypotheses have been put forth. Some geophysicists 49 00:02:50,280 --> 00:02:53,200 Speaker 1: think gets generated by the ceaseless pounding of ocean waves 50 00:02:53,280 --> 00:02:56,400 Speaker 1: onto continental slopes. Others believe that it could be the 51 00:02:56,400 --> 00:03:00,320 Speaker 1: product of atmospheric turbulence and global wind patterns. But if 52 00:03:00,400 --> 00:03:04,279 Speaker 1: that second explanation is true, we'd expect the rumblings amplitude, 53 00:03:04,400 --> 00:03:07,320 Speaker 1: that is, its loudness, to vary from season to season. 54 00:03:08,000 --> 00:03:10,800 Speaker 1: Previous studies have claimed that this is happening, yet the 55 00:03:10,800 --> 00:03:14,960 Speaker 1: new research says otherwise. Dean's group confirmed that the hums 56 00:03:15,000 --> 00:03:18,080 Speaker 1: pitch rises and falls, with its maximum volume hitting a 57 00:03:18,120 --> 00:03:22,080 Speaker 1: frequency of four point five milliherts. That's about ten thousand 58 00:03:22,120 --> 00:03:26,280 Speaker 1: times softer than the faintest noises our ears can detect. However, 59 00:03:26,320 --> 00:03:29,800 Speaker 1: according to the team's findings that amplitude changes don't correlate 60 00:03:29,840 --> 00:03:33,559 Speaker 1: with seasonality. Thus, Dean and her colleagues argue that atmospheric 61 00:03:33,600 --> 00:03:36,800 Speaker 1: issues alone cannot account for the existence of these permanent 62 00:03:36,800 --> 00:03:40,760 Speaker 1: free oscillations. They also think that their research could open 63 00:03:40,760 --> 00:03:44,360 Speaker 1: the door for future research on the Earth's interior. Geologists 64 00:03:44,440 --> 00:03:47,440 Speaker 1: use a process called tomography to map out the inside 65 00:03:47,440 --> 00:03:49,960 Speaker 1: of our world. Think of it as a large scale 66 00:03:50,040 --> 00:03:53,480 Speaker 1: m r I scan. Dean explains that scientists invert the 67 00:03:53,520 --> 00:03:57,200 Speaker 1: recordings of seismic waves to decipher the makeup of various 68 00:03:57,280 --> 00:04:01,560 Speaker 1: layers and structures within the planet. Going forward, ocean bottom 69 00:04:01,600 --> 00:04:05,040 Speaker 1: sizedmometers like those used in her recent study should give 70 00:04:05,080 --> 00:04:08,600 Speaker 1: tomographers more data to work with. Hopefully we'll soon have 71 00:04:08,640 --> 00:04:16,480 Speaker 1: a better idea of what lies beneath our feet. Today's 72 00:04:16,520 --> 00:04:19,320 Speaker 1: episode was written by Mark Bancini and produced by Tyler Clang. 73 00:04:19,600 --> 00:04:21,919 Speaker 1: For more in this and lots of other earth shaking topics, 74 00:04:22,000 --> 00:04:35,680 Speaker 1: visit our home Planet, how Stuff Works dot Com.