Rare forms of atoms, like carbon-13, carbon-14, and nitrogen-15, have long been used to figure out the ages of ancient artifacts and probe the nuances of prehistoric food chains. The source of these rare isotopes? Complicated cascades of subatomic reactions in the atmosphere triggered by high-energy cosmic rays from outer space. Now, a team of scientists is adding one more isotope initiator to its list: lightning. Carbon-13, one tracer used to probe a variety of geochemical processes, is typically formed when high-energy cosmic rays enter the atmosphere and strike the most abundant form of nitrogen there—nitrogen-14 atoms. The atoms lose a neutron, and the unstable nitrogen-13 atom left behind sheds a neutrino and a positively charged electron, or positron. The reaction (and the subsequent annihilation of the positron when it collides with a negatively charged electron) produces a stable carbon-13 atom and two gamma rays with a very particular energy—often used to detect cosmic rays. But back in February, scientists observing an afternoon thunderstorm off the northwestern coast of Japan picked up the same signals. What’s more, the team also detected the wider range of gamma rays given off by unstable nitrogen-15 atoms created when free neutrons slammed into nitrogen-14 atoms. That means that strong bolts of lightning can unleash the same flurry of nuclear reactions as cosmic rays, the researchers report today in Nature. But, they add, the isotopes created by these storms likely constitute a small portion of all such atoms—so the new findings are unlikely to change the way other scientists use them for dating and geotracing.