For 15 Years Sweden Thought Enemy Submarines Were Invading Its Territory. It Turned Out To Be Herring Farts

What could make small bubbles on a scale that made Sweden think it was under threat from foreign invaders, researchers pondered. u3d/Shutterstock.com

It's perfectly feasible that in the 1980s a major diplomatic incident between nuclear superpowers could have been triggered by fish farts. In fact, Russia and Sweden nearly came to blows over this very thing. They just didn't know it at the time. 

Before we move on to farts, first, some background. In 1981, a Soviet submarine ran aground on the south coast of Sweden, just 10 kilometers (6.2 miles) from a Swedish naval base. The Soviets claimed that they were forced into Swedish territory by severe distress, and later navigation errors, while Sweden saw it as proof that the then Soviet Union was infiltrating Swedish waters. It didn't help that when Swedish officials secretly measured for radioactive materials using gamma-ray spectroscopy, they detected what they were 90 percent sure was uranium-23 (used for cladding in nuclear weapons) inside the sub, indicating that it may be nuclear armed.

The submarine was returned to international waters, but the Swedish government remained alert, convinced that Russian subs could still be operating near their territory. Which is when they started to pick up elusive underwater signals and sounds. In 1982, several of Sweden's subs, boats, and helicopters pursued one of these unidentified sources for a whole month, only to come up empty-handed.

This continued for over a decade. Every time they picked up an acoustic signal they would search and find nothing but for a few bubbles on the sea's surface. Sweden was, of course, worried about the intrusions, and couldn't think why, with the Cold War now over, Russia would continue to provoke them in this manner.

But it was farts.

In 1996, Magnus Wahlberg, a professor at the University of Southern Denmark, became involved in the investigation of the strange signals.

"We were brought into this very secret room under the naval base of Bergen in Stockholm," he explained in a TEDx Talk in 2012. "We were sitting there with all these officers and they were actually playing these sounds for us. It was the first time any civilian heard the sound."

He had been imagining it to sound like the ping you hear in films when a submarine is detected or even the noise of a propeller.

"It was nothing at all like that," Wahlberg said. "It sounded like someone frying bacon. Like small air bubbles releasing underwater."

He and a colleague began the task of figuring out what could be making bubbles on a scale that would make Sweden think it was dealing with a nuclear submarine.

"It turns out herring have a swim bladder... and this swim bladder is connected to the anal duct of the fish," Wahlberg said. "It's a very unique connection, only found in herring. So a herring can squeeze its swim bladder, and that way it can blurt out a small number of bubbles through the anal opening."

In layman's terms, they let one rip. Herrings swim in gigantic schools that can reach several square kilometers and up to 20 meters (65 feet) deep. When something near them frightens them – say, a hungry school of mackerel or a submarine on the lookout for Russian spies – they can generate a lot of gas.

To test his theory, Wahlberg bought a herring from a store and applied pressure, and sure enough, it made a sound. He took the footage to the navy personnel and played it back to them. It was a perfect match for the noise they had been hearing.

The good news was that Sweden wasn't under threat from Russia, the bad news was it had spent 10 years deploying its military in pursuit of fish farts. Since it figured out what was and wasn't fish farts, there have been zero reports of hostile intruders in Swedish waters.

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