For sea turtles, there are few habitats more perfect than the cool Pacific water around verdant Enewetak Atoll, halfway between Australia and Hawaii.

Perfect, that is, except for the radiation that pervades it. After capturing the atoll during World War II, the United States tested nuclear weapons there 43 times, then buried the resulting radioactive waste in a concrete tomb that has since begun to leak.

Now scientists have discovered the waste’s nuclear signature in the shells of sea turtles living in the surrounding waters, making the turtles one of a string of animals that are affected by global nuclear contamination.

From tropical oceans to the forests of Germany and the mountains of Japan, radiation from nuclear testing and disasters is showing up in fauna around the world. While these animals’ radiation generally doesn’t threaten humans, they are a testament to humanity’s nuclear legacy.

“It’s a cautionary tale,” says Georg Steinhauser, a radiochemist at the Vienna University of Technology and an expert on animal radioactivity. “Nature does not forget.”

Sea turtles of Enewetak Atoll

Much of the world’s radioactive contamination comes from tests conducted by world powers racing to develop powerful weapons during the 20th century. The U.S. tested nuclear weapons from 1948 to 1958 on Enewetak Atoll.

In 1977 the U.S. began to clean the atoll of radioactive waste, most of which is buried in concrete on one of the islands. Researchers from the study into turtles’ nuclear signatures speculate that the clean-up disturbed contaminated sediments that had settled in the atoll’s lagoon. They believe this sediment was then swallowed by the turtles while swimming, or it affected the algae and seaweed that make up large parts of sea turtle diets.

The sea turtle studied in the paper was found just a year after the cleanup began. Traces of radiation in those sediments made their way into the turtle’s shell in layers scientists could measure, says Cyler Conrad, a researcher at Pacific Northwest National Laboratory who led the study.

Conrad likened the turtles to “swimming tree rings,” using their shells to measure radiation the same way rings in a tree trunk record its age.

“I did not have a full appreciation for how widespread those nuclear signals are in the environment,” says Conrad, who also studied turtles with signs of human-related radiation in the Mohave desert, the Savannah River in South Carolina, and Oak Ridge Reservation in Tennessee. “So many different turtles at so many different sites were shaped by nuclear activity that occurred at those locations.”

Wild boars of Bavaria, Germany

Weapons tests also spread contamination by shooting thick swells of radiated dust and ash called fallout into the upper atmosphere, where it can circle the planet and settle in distant environments.

In the forests of Bavaria, for example, some wild boars occasionally hold staggering levels of radiation. Scientists long assumed that fallout was produced by the catastrophic 1986 meltdown of the Chernobyl Nuclear Power Plant in nearby Ukraine.

In a recent study, however, Steinhauser and his team found that up to 68 percent of contamination in Bavarian boars came from global nuclear testing—conducted anywhere from Siberia to the Pacific. By finding the “nuclear forensic fingerprint” of different isotopes of cesium, some of which are radioactive, Steinhauser’s team ruled out Chernobyl as the source of contamination.

The boars became contaminated through eating truffles, which absorbed radiation from nuclear fallout that settled into the nearby ground.

Steinhauser studied samples of wild boars, usually from their tongues, finding 15,000 becquerels of radiation for every kilogram of meat. These numbers far exceeded the European safety limit of 600 becquerels per kilogram.

When the first results came back, one of Steinhauser’s PhD students said: “These must be wrong … It’s not possible that there’s this much weapons cesium in the wild boars,” he recalls. It was only after they checked the measurements again that they concluded that the “boars are carrying way more old nuclear weapons fallout cesium than they were supposed to be.”

Reindeer of Norway

Chernobyl’s effects are clearly observed elsewhere in Europe. This disaster sent fallout billowing across the continent, leaving a radioactive legacy that stretches into the present. “Europe is heavily contaminated by Chernobyl. It’s our number one source of radioactive cesium,” says Steinhauser.

Much of that fallout was blown northwest to Norway, where it fell in raindrops. Because the fallout’s path depended on unpredictable weather, “the contamination over Norway from the accident is not evenly distributed,” explains Runhild Gjelsvik, a scientist at the Norwegian Radiation and Nuclear Safety Authority. “It’s very patchy.”

The fallout was absorbed by mushrooms and lichens, the latter of which Gjelsvik says are vulnerable to fallout because they lack root systems and absorb their nutrients from the air. These were then eaten by herds of reindeer. Immediately after the Chernobyl accident, meat from some reindeer had radiation levels of more than 100,000 becquerels per kilogram.

Nowadays, says Gjelsvik, much of the contaminated lichen has been grazed away, meaning radioactivity in most Norwegian reindeer is below the European safety standard. But some years, when wild mushrooms grow in higher numbers than normal, the mutton samples can show spikes to as much as 2,000 becquerels.

“Radioactive substances originating from Chernobyl are still being transferred from soils to mushrooms, plants, animals, and people,” Gjelsvik says.

Macacques of Japan

In Japan, a similar problem plagues red-faced monkeys.

After the catastrophic meltdown of the country’s Fukushima Daiichi Nuclear Power Plant in 2011, the concentration of cesium in nearby Japanese macaques rocketed upwards to a maximum of 13,500 becquerels per kilogram, according to a study led by Shin-ichi Hayama, a professor at Nippon Veterinary and Life Science University.

According to Hayama’s research, which mainly focused on tissue samples from the macaques’ hind legs, they likely absorbed the contamination by eating buds and bark on local trees, as well as other foods like mushrooms and bamboo shoots, all of which take in radioactive cesium from the ground.

The high concentrations of cesium, which have declined over the last decade, led Hayama to speculate that monkeys born after the accident may have experienced delayed growth and had smaller heads.

Are these animals dangerous?

The scientists who study radioactive animals emphasize that it is highly unlikely the radiation they contain would ever threaten humans. Some, like the Fukushima macaques, are not eaten and therefore are no threat. Others, like sea turtles, contain so little radiation that they pose no danger.

Others, like Bavarian boars and Norwegian reindeer, are monitored to ensure unsafe meat does not go to consumers. “The regulatory limits are super strict,” says Steinhauser. Nonetheless, these findings have “enormous implications,” he adds. “For many years, we’ve been happy with assuming that nuclear fallout is going somewhere else. But ‘somewhere else’ does not mean it’s lost.”