Lecture: Sounds from the deep sea: Narrator: Listen to part of a lecture in an oceanography class. Professor: For several decades now, we've been picking up all sorts of sounds from the deep sea with hydrophones, that network of underwater microphones I mentioned. Of course, a lot of these sounds have been identified, sounds made by sea animals, movements of the Earth's crust, ships and submarines. The list goes on. But there's mysterious sounds too, sounds where we're not sure of the source. Some last a few minutes. Others go on for years. We don't know if they're biological, geological, or human-made. But it's important to find out and a lot of effort has been spent doing just that. When a mysterious sound is first detected, it's given a name. Like ... there's one called Upsweep. It's a flat tone, very low in pitch, accompanied by a rising tone. Upsweep was heard continuously between 1991 and 1994. And for some reason it got louder during the last fifteen months of that period. At first, we thought Upsweep was some sort of whale song. But that couldn't be, because the sound was detected on both sides of the Pacific Ocean simultaneously. No whale could possibly make a sound that loud, loud enough to carry clear across the ocean. Also, Upsweep's total pattern didn't vary, whereas whale songs change seasonally as the mammals migrate. Female Student: Did we ever figure out the source? Professor: Well, some people think it came from an underwater volcano, like from gas bubbling out of a crack in the sea bed or a stream of lava coming into contact with sea water. The best evidence for this hypothesis comes from seismic studies. Geologists used a seismometer to trace Upsweep's port of origin. Now, seismometers are normally associated with measuring the power of earthquakes. But earthquakes aren't the only thing that sends waves and motions through the Earth's crust. Seismic waves can be created when underwater sound waves hit a solid object like an island. So using seismometers, geologists were able to trace Upsweep's origin to the southern Pacific about halfway between New Zealand and Chile. A research ship was dispatched to this area and found indeed that there were volcanoes there. But we still don't know if it was lava or gas that was making the sound. Or something we haven't thought of yet. Another mysterious sound is named Slowdown. Slowdown has been detected a few times every year since 1997, both in the Atlantic and Pacific Oceans. It's been described as sounding like an airplane. Some people think that like Upsweep, Slowdown emanates from an underwater volcano. But because Slowdown is in the Southern Hemisphere, which suggests the Antarctic region, one researcher, Christopher Fox – he's the director of the Major Acoustic Monitoring Project – well, Fox thinks it's from ice, glacial ice. Slowdown's spectrogram, spectrogram's sort of a graph of sound frequencies, Slowdown's spectrogram looks a lot like the spectrogram made by rubbing your fingers over a piece of paper, friction basically. So, Fox hypothesize that Slowdown is coming from a glacier sliding across a piece of the Antarctic continent. In fact, we tried to correlate the timing of Slowdown with the occurrence of known ice events, like a huge chunk breaking off. He didn't come up with anything but that doesn't necessarily mean he's wrong. And if it is ice rubbing on land, well, that's important information, even more important than using sounds to find underwater volcanoes. It would be further evidence that the Antarctic ice sheet is breaking off. So the study ocean acoustics could add to our knowledge base of climate change. A challenging aspect of ocean acoustic research is that most sounds occur at very low frequencies. We can't hear them by putting on headphones and just listening. What researchers have to do is to record what may or may not be silence. Then they speed up the recording. If there's a low frequency sound there, then we will hear it on the speed it up playback. I will leave you with another more mysterious sound, one called Bloop. We think it's made by an animal because it consists of a rapid variation of frequency. But it was detected by hydrophones that were placed very far apart. So, if there's some creature out there larger than the largest whale, or something just far more efficient at making sounds, maybe one of you will eventually solve the puzzle.