Did you know Baltic herring nearly escalated Cold War tensions?

Did you know that Baltic herring almost escalated Cold War tensions and brought Sweden and the Soviet Union to the brink of diplomatic crisis? 

The Cold (Water) War

Picture this: it's 1981, and tensions between East and West are at their peak. A Soviet submarine runs aground off the Swedish coast near Karlskrona, just 10 kilometres from a major naval base. The Soviets claim navigation errors, but Swedish officials detect what they believe is uranium-235 (used in nuclear weapons) aboard the sub. The submarine is eventually escorted back to international waters, but Sweden remains on high alert, convinced that Russian submarines are conducting covert operations in their territorial waters.

What happened next would define Swedish naval operations for the next 15 years. The Swedish navy began detecting mysterious underwater acoustic signals throughout the Baltic Sea – sounds they described as resembling "frying bacon" with distinctive popping and hissing characteristics. These weren't the typical sonar pings you might expect from submarines; they were far more subtle and perplexing.

Sweden launched some of the most intensive submarine hunts in Cold War history. In one operation alone, several Swedish submarines, surface vessels, and helicopters pursued these elusive acoustic signatures for an entire month, spending enormous resources but finding nothing except occasional bubbles on the sea surface.

This pattern continued for over a decade. Every time Swedish hydrophones detected these "typical sounds," as the navy termed them, expensive search operations would be launched. Sweden's Prime Minister Carl Bildt even wrote to Russian President Boris Yeltsin in 1994, demanding explanations for these continued intrusions. Yeltsin's response was diplomatically equivalent to "we have no idea what you're talking about".

The Swedish military was convinced they were dealing with a new generation of ultra-quiet Soviet submarines – so advanced that they could penetrate Swedish waters undetected yet still produce these mysterious acoustic signatures.

The breakthrough

In 1996, the Swedish navy brought civilian scientists into their acoustic surveillance facility beneath the naval base in Stockholm. For the first time, non-military personnel would hear the sounds that had mystified Sweden's defence establishment for 15 years.

Professor Magnus Wahlberg from the University of Southern Denmark and his colleague Håkan Westerberg were the chosen experts. In his 2012 TEDx talk, Wahlberg described the surreal experience: "We were brought into this very secret room under the naval base... 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".

The scientists initially expected to hear typical submarine noises – the ping of sonar or the whir of propellers. Instead, they heard something completely different: sounds exactly like someone frying bacon, created by small air bubbles releasing underwater.

The what?

What Wahlberg and Westerberg discovered revolutionised our understanding of marine biology and prevented a potential international incident. Baltic herring (Clupea harengus) possess a unique anatomical feature: their swim bladder is connected to both their alimentary canal and anal opening.

This allows them to release gas bubbles through their anal opening – essentially, they can fart.
When herring are startled, during vertical migration, or for communication purposes, they squeeze their swim bladders and expel small gas bubbles. The sound produced is called a "pulsed chirp" – lasting 32-133 milliseconds and consisting of 7-50 transient pulses with decreasing frequency emphasis.

Baltic herring congregate in massive schools that can extend several square kilometres and reach depths of 20 meters. When thousands of these fish simultaneously release gas bubbles – whether from fear, communication, or routine behaviour – the collective acoustic signature is remarkably similar to the sound produced by submarine engines and sonar systems.

This discovery highlighted both the ingenuity of biological systems and the limitations of 1980s sonar technology. The Swedish navy's hydrophones were sophisticated enough to detect these subtle acoustic signatures across vast distances, but lacked the analytical capabilities to differentiate between biological and mechanical sources.

European innovation in maritime defence

Today's European underwater surveillance systems are hugely more advanced. The European Defence Agency has launched the ETLAT II programme, investing €4 million over three years to develop next-generation "Thin Line Array" technologies for underwater surveillance. These systems combine advanced numerical modelling, vibration isolation, and experimental validation to achieve unprecedented accuracy in submarine detection.

Modern AI-powered sonar systems can now differentiate between various underwater acoustic sources with remarkable precision. NATO's DIANA accelerator programme is supporting startups developing biomimetic underwater sensing technologies, including systems that replicate the electrical sensing capabilities of certain fish species.

Current European underwater surveillance programmes incorporate artificial intelligence, quantum sensing, and advanced materials science help ensure that future acoustic “anomalies” can be properly identified without triggering diplomatic crises or expensive search operations.

Sources: For 15 Years Sweden Thought Enemy Submarines Were Invading Its Territory—It Turned Out to Be Herring Farts, IFLScience (https://www.iflscience.com/for-15-years-sweden-thought-enemy-submarines-were-invading-its-territory-it-turned-out-to-be-herring-farts-57782); How herring farts almost lead to a diplomatic crisis, TEDx (https://www.youtube.com/watch?v=xQ1Jr6QqSlE); Sounds produced by herring (Clupea harengus) bubble release, Academia.edu (https://www.academia.edu/25843210/Sounds_produced_by_herring_Clupea_harengus_bubble_release); ETLAT II develops advanced underwater surveillance technology, European Defence Agency (https://eda.europa.eu/news-and-events/news/2024/12/17/etlat-ii-develops-advanced-underwater-surveillance-technology).