A Cold War Mystery: How Herring Fooled the Swedish Navy

The Cold War was an era defined by tension, suspicion, and constant surveillance. Around the world, intelligence personnel monitored radar screens and intercepted signals around the clock, watching for anything that might indicate an approaching threat. In an atmosphere where a misunderstood signal could have enormous consequences, even the strangest sounds demanded attention.

For Sweden, one of the most persistent mysteries came not from the sky, but from beneath the sea.

Mysterious Signals Beneath Swedish Waters

In the early 1970s, the Swedish Navy began detecting unexplained underwater sounds. The sea is naturally filled with noise, much of which trained specialists can identify. These signals, however, were different.

They were rhythmic and repetitive, almost reminiscent of coded transmissions, yet they matched no familiar pattern. Given the political climate, suspicion quickly fell on Soviet submarines.

The Navy investigated, conducting searches and mapping the areas where the sounds had been detected. Nothing was found. Eventually, operations returned to normal—but the mysterious signals did not disappear.

Again and again, the sounds returned. Their rhythm seemed deliberate, yet their acoustic signature remained unfamiliar. Over time, the phenomenon became a recurring concern for Swedish defense officials.

Then, in 1981, Sweden received dramatic confirmation that Soviet submarines really were entering its waters. A Soviet submarine ran aground inside Swedish territorial waters, triggering a serious diplomatic crisis.

After the incident, Sweden dramatically increased its underwater surveillance. Ships, helicopters, sonar equipment, and networks of hydrophones were deployed to detect possible intrusions.

And the mysterious sounds continued.

A Mystery That Lasted for Years

To Swedish defense officials, the explanation seemed obvious: Soviet submarines were somehow entering Swedish waters without being detected visually.

Analysts studied the recordings and tried to understand the apparent movements behind them. Acoustic signatures seemed to split, multiply, move, and sometimes merge again. On sensitive equipment, the sounds appeared consistent with objects moving beneath the water.

Significant resources were devoted to investigating the suspected intrusions, and the mystery continued for years.

Then the Soviet Union collapsed.

The underwater sounds, however, did not stop.

With the Cold War over, the urgency surrounding the mystery began to fade. But the question remained: If Soviet submarines were responsible, why were the strange signals still being detected?

The Unexpected Culprit

In 1996, civilian marine biologists were invited to examine the recordings. Among them was marine biologist Magnus Wahlberg, who recognized something crucial: The sounds did not appear to be mechanical at all.

They were biological.

The unlikely source was herring.

Herring have swim bladders that help them control buoyancy, and they can release gas through a duct connected to the digestive system. This produces short clicking sounds. When large schools of herring produce these sounds, sensitive underwater equipment can pick up a surprisingly complex series of acoustic signals.

What military analysts had interpreted as evidence of sophisticated underwater activity could, in some cases, be explained by schools of fish.

The sensors themselves had detected real sounds. The problem was the interpretation. In an environment already shaped by genuine Soviet submarine activity, unfamiliar underwater noises were naturally viewed through the lens of a possible military threat.

When Fish Fooled a Navy

The story later became famous well beyond Sweden. Research into the unusual sounds produced by herring earned an Ig Nobel Prize in 2004, an award celebrating research that first makes people laugh or wonder—and then think.

Importantly, Sweden's fears during the Cold War were not entirely unfounded. Soviet submarine incursions into Swedish waters really did occur, and the 1981 grounding provided undeniable evidence of that threat.

But at least some of the mysterious underwater signals that caused years of concern had a far less alarming explanation.

The episode is a fascinating reminder of how strongly expectations can influence interpretation. When everyone is searching for submarines, an unfamiliar underwater sound can begin to sound very much like a submarine.

And the humble herring? We may know it best as a familiar fish on the dinner table. But for years, its surprisingly noisy underwater behavior helped create a Cold War mystery that kept an entire navy guessing.