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When in 1981, during the Cold War, a Soviet submarine collided with rocks near a Swedish naval base, it caused a panic in Sweden that would last for years.

The neutral country was suddenly on red alert and the concerned government ordered that emergency funds be dedicated to detecting submarines.

The hunt for ships infiltrating its territorial waters became a serious matter … depth charges were launched, underwater mines detonated.

But even after the end of the Cold War, the submarine detection system continued to activate. And Sweden blamed Russia.

In 1995, a new Swedish government decided to establish a commission to investigate.

“It was all very secret because that typical sound was the main evidence of the infiltration,” recalls, in conversation with the BBC, Håkan Westerberg, a Swedish scientist, who at the time was using acoustics to track and investigate fish populations.

“I was asked to investigate whether there was a biological explanation for the typical sound.”

Scientists to the water!

“After several investigations, it turned out that this cavitation sound was produced by minks swimming: someone happened to see them the moment they made the recording.

“I’ve heard that sound and can really understand being mistaken for a propeller because when minks swim, they flap their four legs and sound like four blades.”

The enemy turned out to be a mink.

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They were among the first civilians to hear the secret recording of the mysterious noise.

“It sounds a bit like a flock of birds sitting on a power cord chatting or like when you fry bacon … it’s a multitude of small chorus sounds,” describes the scientist.

But why would such a sound make them think it was a submarine?

secret propulsion mechanisms“Westerberg replies, laughing.” data-reactid = “90”> “That’s a good question as it doesn’t sound like the sound of a propeller but they had the idea that it was secret propulsion mechanismsWesterberg replies, laughing.

They thought that the typical sound was perhaps caused by many small bubbles, but it was still a puzzle.

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Biologists were also given access to top-secret data on where and when the recordings had been made, and Westerberg noted that they always seemed to be made during the day and moved from offshore to offshore, depending on the season.

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That seemed strange to him. And suspiciously similar to the movements of something unexpected: Sweden’s favorite fish, herring.

Traditional Swedish summer food … with herring, of course.

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Could the herring make a noise similar to the typical sound?

to adjust your floating ability. But no one had studied the subject in detail. “Data-reactid =” 120 “>” Herrings are normally silent fish, but we knew that they have a swim bladder that has an opening in the anus; can expel air to adjust your floating ability. But no one had studied the subject in detail.

“Investigating, we found observations that when killer whales attacked schools of herring, all fish started to bubble, which is probably a very efficient defense mechanism,” explains the biologist.

Could it be that the herrings were farting en masse?

Not very elegant but …

There were no recordings of that phenomenon, but Westerberg and Walberg had a fish tank and hydrophone to record the noise, so they decided to do an experiment.

“We bought some herrings and put them in the tank with the hydrophone and we squeezed them. We saw and recorded the sound of the bubbles coming out of the fish’s anus.”

The problem was that the noise had the correct profile but the wrong frequency. The reason was that the tone and frequency of the bubbles change depending on the depth. A herring fart in the sea sounds different than one in a small tank. After making the necessary adjustments, the new sound coincided.

but it was difficult to convince them“.” data-reactid = “127”> “Each bubble made a separate little sound that was exactly what you heard when analyzing the typical sound in detail. So we sent a video and recording of that to the Navy specialists, but it was difficult to convince them

Defense mechanism or simply fright … the point is that in some cases, the silent herrings make noise.

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However, to convince the Navy they had to find evidence so – and this gives you an idea of ​​how serious the whole thing was – Westerberg was loaded onto a Swedish submarine to find a herring bank and trigger the production of such bubbles. .

“I had fun … I spent a week in a submarine chasing herrings, not with much success, although I was also sometimes bored of sitting underwater.”

“It was difficult for those who had been detecting submarines for 10 years to admit that it was a herring hunt. The Navy never publicly acknowledged it, it just stopped using the typical sound as evidence.”

And almost overnight, reports of Russian submarines venturing into Swedish territorial waters faded.

Deep sounds

How – you might ask – can a small animal, furry or scaly, be mistaken for a massive submarine.

The problem is that sound travels long distances underwater. If you don’t know where it’s coming from, something small that moves slowly close by may sound like something much bigger and faster by far.

They were not as hostile as they sounded.

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Westenberg and Walberg had inadvertently helped ease diplomatic tensions, but were unable to reveal what they had discovered for a few years. However, thanks to that research, both received an Ignoble award.

Despite the fact that history shows that not everything is as hostile as it might seem, Westemberg feels a certain sympathy for the navy. There are, he says, many mysterious sounds from the deep that can confuse your mind.

there are mysterious things in the depths of the sea“.” data-reactid = “178”> “I’ve heard so many funny things with hydrophones, especially at night. Fish growl and squeal … they make a lot of noise. It’s easy to think that there are mysterious things in the depths of the sea