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Calls from the deep
15 June 2002
From New Scientist Print Edition.

David Wolman

Even without the beard, Christopher Fox would look nothing like Jodie Foster. Yet as he sits in his office trying to zero in on mysterious sounds emanating from far away, I'm reminded of the scene in Contact where Foster is sitting on her car, wearing headphones and listening for a message from outer space.

Only in this case, Fox is tuned in to unidentified noises from the deep ocean, sounds with strange names like Train, Whistle, Slowdown and Bloop. "Just when you think there're no mysteries left in this world, we have a whole bucket of them on the screen here," says Fox, who directs the US National Oceanic & Atmospheric Administration's Acoustic Monitoring Project at the Pacific Marine Environmental Laboratory in Newport, Oregon.

Earth's oceans are full of noise: boats, whales, submarines and earth tremors all add to the aquatic cacophony. The study of ocean acoustics has helped scientists monitor whale communication and migration, pinpoint the locations of undersea earthquakes and volcanoes, and even measure ocean temperatures, yet there's still a handful of sounds that continue to baffle researchers. They are generally very low-frequency and must be speeded up to be audible. Some last just a few minutes, while others go on continuously for years at a time. Nobody knows for sure what causes these sounds, but suspicion has fallen on volcanoes, icebergs and even huge, undiscovered beasts.

The system that picks up all these signals is a military relic. In the 1960s the US Navy set up an array of underwater microphones, or hydrophones, scattered around the globe. Known as SOSUS, short for Sound Surveillance System, this array was intended to track Soviet submarines. But the end of the cold war, and probably the development of superior surveillance technology, meant that by 1991 SOSUS could be shared with civilian scientists.

These listening stations sit on the seabed at a depth where sounds can travel for thousands of kilometres. Hundreds of metres below the ocean surface, sound waves become trapped in a layer of water known as the deep sound channel. Here the temperature and pressure are such that when sound waves try to move outside this layer, they get bent back into it. So they just keep on travelling. "It prevents the sound from being scattered by either the ocean surface or the ocean bottom, so sound just progresses along merrily without ever hitting anything," says Robert Spindel, a physicist at the University of Washington in Seattle.

Ocean sounds are analysed by examining their characteristics as they arrive at different hydrophones. Fox says he can identify most of the sounds recorded from the deep by their spectrogram alone. "The sound waves are almost like voice prints. You're able to look at the characteristics of the sound and say, 'There's a blue whale, there's a fin whale, there's a boat, there's a humpback whale and here comes an earthquake.'" But many sounds remain unidentified.

Most of these signals have names that Fox came up with on a whim. As well as Train, Whistle and Bloop, there's even the Gregorian Chant. The names aren't meant to indicate the likely origin of the sounds, of course, as no one knows what or who is responsible for most of them. But in a few cases the real perpetrators may soon be identified.

Take the strange sound called Upsweep. It's a flat tone of only a few hertz accompanied by rising tones. It was heard continually between 1991 and 1994, its loudest during the last 15 months. During decades of tuning in to the oceans, the US Navy had never seen this signal before.

It was first believed to be biological, possibly produced by fin whales. But when the sound was picked up by receivers on opposite sides of the Pacific, researchers concluded that it was too loud to have been produced by a whale. The signals also stayed the same over the course of many seasons, whereas whale song should have varied as the whales migrated.

Then in 1996, geologists working in Tahiti came up with the most plausible explanation so far. Emile Okal of Northwestern University in Chicago and Jacques Talandier, formerly with the French Atomic Energy Agency, used seismometers to get a handle on Upsweep. This technique was possible because when sound waves in water hit a mass such as an island, the mass acts like a giant hydrophone, converting the sound into seismic waves.

Like Fox, Okal realised Upsweep couldn't have come from whales, but for a different reason: the sound was too uniform, he says. Whale song usually has more changes in pitch and amplitude, simply because it's hard to communicate much with little variation in tone. Talandier and Okal suspected the sound was instead caused by a volcanic process. Although Upsweep's relatively pure tone didn't fit with the more varied sounds typical of other volcanic activity, the researchers speculated that it came from some kind of oscillation of bubbly liquid, perhaps seawater coming into contact with a large pool of lava. "Somehow within the volcanic cycle," explains Okal, "there must be some resonance of a column of water or gas."

Okal and Talandier homed in on the source with a beefed-up form of triangulation, using readings from eight different directions, including SOSUS data provided by Fox. It all pointed to a spot south-west of the Eltanin fracture zone in the remote southern Pacific, roughly halfway between New Zealand and Chile. They radioed a French research vessel in the region, which headed to the place and found that a previously identified chain of seamounts were in fact volcanic.

Other mystery sounds may have more straightforward origins. Many noises can be traced to weather and ocean currents, and Fox suspects these are also responsible for the sound known as Train, which resembles the rushing noise of a distant train. "Moving fluids generate vibrations, just like blowing air through a clarinet," Fox says. "If you have moving ocean water and the right conditions coming around a seamount or something, that could generate sound."

A far more romantic possibility for the source of mystery noises is marine life. We haven't yet catalogued the sounds produced by many creatures—little is known about the calls of the sei whale, for example. Even whales whose calls are well studied could be responsible for a mystery sound or two, because most whale research focuses on audible frequencies rather than the lower frequencies that ocean hydrophones pick up.

Fox's hunch is that the sound nicknamed Bloop is the most likely to come from some sort of animal, because its signature is a rapid variation in frequency similar to that of sounds known to be made by marine beasts. There's one crucial difference, however: in 1997 Bloop was detected by sensors up to 4800 kilometres apart. That means it must be far louder than any whale noise, or any other animal noise for that matter. Is it even remotely possible that some creature bigger than any whale is lurking in the ocean depths? Or, perhaps more likely, something that is much more efficient at making sound?

In my mind, the suggestion of huge ocean creatures raises a vision of giant squid. There are no confirmed sightings of giant squid in the wild, though dead ones have washed up on beaches, and whales sometimes bear telltale sucker-shaped scars. "We don't have a clue whether they make noise or not," says Fox.

Phil Lobel, a marine biologist at Boston University, agrees that Bloop is likely to be biological in origin but he's not keen on the squid idea. "Cephalopods have no gas-filled sac, so they have no way to make that type of noise. Though you can never rule anything out completely, I doubt it." Lobel thinks the sound is louder than any other whale noise, because I've heard presentations about whale song detected at that distance and farther."

Perhaps the most tantalising sound is Slowdown, which has been picked up a few times every year since about May 1997. "It sounds like an airplane going by," says Fox. It has been detected in the Atlantic as well as the Pacific, but always from the south, indicating an Antarctic origin. But the source is so far from any of the SOSUS hydrophones it is not yet possible to pinpoint more specifically where it's coming from, let alone work out what's making it.

In order to rule out an obvious possibility, Fox consulted the US Navy. At the National Maritime Intelligence Center in Suitland, Maryland, he asked navy specialists if they had, or knew of, any military toys that might be producing these sounds. "I thought maybe there was some weird new device," says Fox. "There are torpedoes and things that have been developed over the last decade, and we should be able to hear them." The answer was no. As far as the military was concerned, the sounds were not man-made.

Two of the navy researchers had a heated argument about the source of the sound, according to Fox. One maintained it had geological origins like Upsweep, while the other was convinced it was coming from Antarctic ice. They declined to comment for this article, but Fox himself favours the ice hypothesis. The sound's southerly origin is one clue, and another was unearthed by one of the navy researchers. He showed Fox a spectrogram that was strikingly similar to Slowdown, except that it was higher pitched and much shorter. "He made me suffer a bit," recalls Fox with a chuckle, "and then finally explained what it was." He had placed a microphone next to a glossy magazine page and then run his fingers over it to create friction. It didn't take Fox long to make the connection: Slowdown had to be caused by a massive friction phenomenon, quite possibly ice rubbing over land.

This finding could have important consequences. The break-up of Antarctic ice is a key indicator of climate change, and although satellite imagery is a powerful weapon for detecting ice motion, it cannot catch every incident. Could ocean acoustics provide a new monitoring tool?

To investigate further, Fox went to the National Ice Center in Washington DC, where sea ice is carefully mapped and monitored for scientific, military and commercial purposes. He tried to correlate Slowdown with the timing of any known iceberg calving event, or possibly a glacial surge. Glaciers usually inch toward the sea, but occasionally they lurch with much bigger, louder movements. Fox drew a blank, but that doesn't mean the theory will be ditched. The next task is to study seismograms from Antarctica and find out whether Slowdown shows up on them. "It's the kind of sound that seismologists don't look at," says Fox. "They look for earthquakes, so it's possible these signals are on seismograms, but nobody's looked at them yet."

And if that doesn't solve the Slowdown mystery, a set of nine hydrophones deployed in the Indian Ocean as part of the treaty banning nuclear weapons tests (see New Scientist, 8 June, p 12) could shed new light on its nature, and also help researchers find out what ocean acoustics can tell them about Antarctic ice, and possibly even climate change.

Meanwhile, at the April meeting of the Seismological Society of America in Victoria, British Columbia, Okal announced a further development. Last year in Polynesia, he recorded a sound that seems to have been generated by a huge iceberg called B-15B, drifting in the Ross Sea near Antarctica. The iceberg, which has an area of about 4000 square kilometres, is ringing like a bell, sending out a deep, pure sound with a frequency of just a few hertz. So what's playing the role of the clapper? It may be the berg cracking, hissing or spouting water, or perhaps it's scraping the ocean floor. Okal says it's too early to tell, but his findings do at least strengthen the idea that Antarctica or Antarctic ice may be behind some of these mystery sounds.

Progress in ocean acoustics is slow, in part because the field is so specialised, but also because research is disjointed. Seismologists do their thing, and geologists, oceanographers and military analysts do theirs. "If we landed a space probe on another planet and recorded these unknown sounds, there would be, I think, significant vigour in the scientific community to find out what they were," says Craig McLean, director of the Ocean Exploration Program at NOAA. With the oceans, he says, people are often inclined to look out across the water and accept that we don't know what lies beneath. But with SOSUS data and a growing number of sound-savvy scientists, that may finally be changing.

David Wolman is a writer based in California

From issue 2347 of New Scientist magazine, 15 June 2002, page 35

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