When a thunderstorm rolled over the Japanese beach town of Uchinada early one December morning in 2015, residents had no idea what they were in for. Sometime after 5 am, a flash of lightning struck a wind turbine and incredible weather phenomena occurred.

While scientists expected the storm, they did not expect the perplexing outcome. The thunderstorm turned into a particle accelerator and blasted gamma radiation at the ground.

Scientists have observed so-called “terrestrial gamma-ray flashes” shooting upwards from the Earth into satellites since the early 1990s. But the 2015 flash was different because not only did it shoot down into the ground instead of up into space, but it confirmed a long-standing suspicion about these flashes by sending a barrage of neutrons into particle detectors a thousand feet away. According to the paper published in Geophysical Research Letters, this is groundbreaking. But that isn’t all.

The flash was massive. “The total fluence of gamma-rays at the ground was calculated to be approximately ~100,000 photons/cm2, much larger than the total combined fluence of all TGFs observed by satellites since 1994,” according to the paper. “This is one of the brightest gamma-ray flashes ever seen on the ground,” Dwyer said.

A team in the United States and Japan led by then-graduate student Gregory Bowers observed these gamma-ray flashes with special particle detectors. Satellites have observed gamma-ray flashes before, but this latest flash was unusual in several aspects, and it came with a prize: They observed a burst of subatomic particles (in this case, neutrons) in their detector. Their observations could shed light on what exactly causes gamma ray flashes and what causes lightning in general, and help researchers get a better handle on how the electric fields inside thunderheads behave. –Gizmodo

“We really don’t understand how thunderstorms make lightning very well,” study author Joseph Dwyer from the University of New Hampshire told Gizmodo. “How it gets started inside the cloud involves how strong the electric fields are. If you get clues to what the electric fields are doing by seeing the gamma rays coming out, that gives you an idea as to what’s going on.”

And as far as “what’s going on,” there are still a few ambiguities scientists are working out. Perhaps the lightning itself is directly responsible for the gamma-ray flashes, or it could be that the electric field inside of the thundercloud begins to accelerate particles, like a particle physics experiment. The interactions in the clouds could have created positrons, a kind of antimatter.  Scientists have documented this happening before. When positrons interact with their regular matter counterpart, (in this case, electrons,) they turn into light particles, called photons, which would show up as gamma rays.

Of course, the neurons also add another layer to this phenomenon. Researchers had predicted that these powerful emissions of neutrons from these flashes occurred, but no one had ever observed them before. They think that high-energy gamma rays interacted with nitrogen in the atmosphere, leading to the production of neutrons. The thunderstorm may literally have become a nuclear reactor, where energy and elements cause reactions in the sky that produce radiation.

This has proven that thunderstorms can turn into natural particle-accelerating nuclear reactors for reasons we don’t quite understand yet, but there’s no need to worry about the radiation.

“You’d probably get more radiation having your tooth x-rayed” than being hit by the neutrons from one of these blasts, said Dwyer. Maybe the dose would be higher for pilots flying around inside the storm, but “this is not my highest concern if I ended up inside a thunderstorm,” he said. “There are a half-dozen ways a thunderstorm can kill you if you’re inside.”