Nuclear weapons tests that took place in the mid-20th century are still a major source of radioactivity in Germany’s wild boars, accounting for anywhere from 10 to 68 percent of contamination in meat samples from these animals, reports a new study.

The discovery could help to explain why wild boars have remained so much more radioactive than other species in their ecosystems, which is a longstanding problem known as the “wild boar paradox.” Previously, scientists assumed this radiation was almost entirely produced by the catastrophic meltdown of the Chernobyl nuclear reactor in 1986, but the new research shows that weapons tests are also a substantial and long-lived source of environmental contamination, a finding that is particularly ominous in light of Russia’s nuclear saber-rattling during its invasion of Ukraine.

Nuclear fallout produces radioactive particles, including isotopes of the element cesium, which can still be found in ecosystems today. Radiocesium has a half-life of 30 years, meaning that half of it decays in that time period, so it makes sense that concentrations of the contaminant have been gradually receding in Europe over time.

Wild boars are the bizarre exception to this rule. Radiocesium levels in these animals have remained constant, a puzzling fact that has rendered them unsafe to eat and has thus contributed to a rampant overpopulation of boars across Europe as demand for their meat has plummeted.

Now, scientists co-led by Georg Steinhauser and Bin Feng, who are radiochemists at the Vienna University of Technology, have discovered that much of this persistent contamination can be traced back to nuclear weapons testing.

The findings suggest that multiple sources of nuclear fallout can “vastly surpass the impact of any singular yet dominant source” and “illustrates that strategic decisions to conduct atmospheric nuclear tests 60−80 years ago still impact remote natural environments, wildlife, and a human food source today,” according to a study published on Wednesday in the journal Environmental Science & Technology.

“I've been obsessed with these wild boars for more than a decade now because of the wild boar paradox,” said Steinhauser in a call with Motherboard that also included Feng. “After Chernobyl, there were many studies on food safety that focused on which food items are still safe to eat and which are more vulnerable to cesium contamination. It was soon found that forest animals in particular are most vulnerable—including deer, hare, and wild boar.”

“For all the other animals, the contamination levels were pretty high in the beginning, but then a few years later, you could already see it going down,” he continued. “This was as everybody expected, because, of course, the radioactivity comes into the animal by eating contaminated foods. The same thing did not take place with boars. They were high in the beginning, and they’ve stayed high for a long time.”

Steinhauser has tried to solve the wild boar paradox many times, including by investigating whether fatty tissues in the boars were hidden reservoirs for radiocesium, but those efforts did not crack this curious riddle. In the new study, his team partnered with hunters to procure samples of wild boar meat from across Bavaria, which the team ultimately examined with sophisticated spectrometers.

The researchers had suspected that they might find lingering contamination from weapons testing in the samples, as this source leaves a different radioactive signature than Chernobyl fallout. However, Steinhauser said it was “mind-blowing” to discover that weapons fallout is such a strong source of contamination in the meat samples. The results suggest that different sources of radiocesium can interact in the soil, potentially extending the lifetime of contamination.

“It’s like a snowball effect,” Feng said. “The sources mixed together, and became a new source that can get stronger. This is the reason, we think, why the cesium contamination is so strong and persistent.”

But why boars, specifically? After all, radioactivity from weapons tests around the world affects everyone. Scientists have determined that wild boars are particularly vulnerable to consuming mixed-source contaminants because they rely on underground food sources, especially truffles, which become radioactive hotspots as cesium sinks into the soil.

“Cesium migrates through soil very slowly, at only a few millimeters per year, and sometimes even less,” Steinhauser said. “As time goes by, truffles seize more of the cesium, but at the same time, less cesium is left because it's decaying away according to its half life. These are two different mechanisms, or two different effects, that cancel each other out so the outcome is constant. It's a constant level over many, many years. That's the bottom line of our current hypothesis that is supported by the study.”

The hypothesis sheds new light on an old paradox, and it also emphasizes the unpredictable impacts of nuclear meltdowns and detonations. The study warned that any future contamination would add to the existing fallout, which would “further aggravate the current contamination situation.”

“According to the International Atomic Energy Agency, 56 nuclear power reactors are currently under construction across the world, thus underscoring the role of nuclear power in the future global energy portfolio,” the researchers wrote in the study. “With the intensifying war between Ukraine and Russia, much concern has been expressed about the terrible consequences of a nuclear war or a combat-triggered nuclear accident.”

“Once released, radiocesium will remain in the environment for generations and impact food safety immediately and, as shown in our study, for decades,” the team added. “Any additional releases will cause further accumulation and mixing with older sources, making it necessary to understand the underlying mechanisms of the biogeochemical cycling of radiocesium.”

To that end, the researchers plan to conduct new studies to resolve longstanding questions about radiocesium contamination, including an effort to definitively link the radioactive signatures found in wild boars to truffles. Understanding how different nuclear contaminants interact with each other, and their broader environment, could reveal the full impact of nuclear disasters of the past, while underscoring the profound risks of future fallout.

“When I analyzed the data, it was really surprising to me at least 60 years ago—when two half-lives have passed—it still affects our ecosystem,” Feng said. “We should be cautious about our use of nuclear energy and I hope that we can protect our environment for the next generation.”