Nearly 100 years ago, physicists proposed that high-energy processes occurring during thunderstorms might trigger nuclear reactions. But limitations in detector technology and difficulties in making measurements during thunderstorms have prevented researchers from recording direct experimental evidence of such nuclear reactions—until now. On Feb. 6, a team led by Teruaki Enoto of Kyoto University tracked two cloud-to-ground lightning strikes during a thunderstorm over the coast of the Sea of Japan. Using data recorded at monitoring stations near the lightning strikes, which include time- and energy-correlated signals of γ rays, neutrons, and positrons, the team concludes that it observed γ-ray-induced photonuclear reactions (Nature 2017, DOI: 10.1038/nature24630). Specifically, the researchers detected telltale signs that γ rays, which are produced in thunderclouds as intense electric fields drive high-energy electrons into airborne molecules, can knock a neutron out of an 14N nucleus, creating an 13N isotope. That isotope is unstable: It decays into a neutrino, a positron, and a stable 13C nucleus. Upon collision with an electron in an atmospheric molecule, the positron and electron undergo annihilation and produce a pair of γ rays with a characteristic energy of 0.511 MeV. The researchers note that related processes generate 14C and 15N isotopes. These findings broaden understanding of atmospheric electrical phenomena and reveal a previously unrecognized natural source of radioactive isotopes on Earth.