Someone Threw a Fermented Fish Bomb at a Swedish Music Festival

Last week at the Uport music festival in Ulricehamn, Sweden, attendees were startled by an explosion that cracked windows, sent shrapnel flying, and left a crater nearly a foot wide on the festival grounds.

The IED in question: a small bomb taped to a can of fish.

Specifically, it was a can of surströmming, a traditional Swedish preparation of fermented Baltic herring that the Telegraph's Harry Wallop once described smelling like "a mixture of old nappies that have been left to linger unsealed at the bottom of a bin, combined with the unmistakable and sharp note of dog faeces." People often eat it outside so that the breeze can carry away some of its signature odors.

No one at the festival was injured, but the impact of the bomb was still felt after the explosion. "Outside there was a huge cloud of smoke and it reeked of gunpowder and fermented herring," festival general Sebastian Andersson told a local paper. Swedish police are apparently still searching for the fish bomber, but haven't gotten far. No one's exactly sure why anyone would want to bomb the festival—unless it was a violent protest of Icona Pop, perhaps—and, of course, the fact that this bomb was made of fish adds another layer of mystery.

Students in the history of surströmming, however, know that cans of the fermented herring are explosive enough by themselves.

Like most fermented foods, surströmming releases gas during fermentation, but that doesn't always stop during the canning process. In fact, both Air France and British Airways have banned the cans of fish as potential explosives, for fear that they might violently rupture mid-air.

While there's no record of anyone dying from an exploding can of surströmming, there've certainly been some close calls. In February, surströmming expert Ruben Madsen was called in to "disarm" a 14-year-old can of the fish that had been tucked under the eaves of a couple's cabin in the Norwegian mountain town of Trysil, as the bulging can reportedly managed to raise the roof by two centimeters. The couple even notified Norway's Armed Forces, just in case of accidental fish-blast.

"There really isn't any risk for an explosion," Madsen said at the time. "Of course, some fermented herring might come spurting out when we open it." When he did crack the can open some days later, however, he found that the fish had completely dissolved in its brine: "Normally surströmming smells really aggressive, but this was worse. It was terrible….It was some kind of rotten porridge."

Though Madsen escaped the hurt locker of a fish bomb, residents in the town of Enaanger, Sweden were less lucky. In May, a bayside warehouse there caught fire, turning the 1,000 cans of surströmming inside into small, stinky rockets. For six hours, the cans continued to explode. One of them cleared an outhouse roof, and another shot straight over the bay and into a neighbor's house.

"The firefighters were surprised," the warehouse owner told Radio Sweden. Luckily, no one was injured, but the area apparently smelled like fried herring for a long time after.