Following the discovery of induced fission by Otto Hahn and Fritz Strassmann in 1938, Soviet physicists Georgy Flyorov and Konstantin Petrzhak began conducting experiments to explore the effects of incident neutron energy on uranium nuclei. Their equipment recorded fission fragments even when no neutrons were present to induce the decay, and the effect persisted even after the equipment was moved 60 meters underground into the tunnels of the Moscow Metro's Dinamo station in an effort to insulate it from the effects of cosmic rays. The discovery of induced fission itself had come as a surprise, and no other mechanism was known that could account for the observed decays. Such an effect could only be explained by spontaneous fission of the uranium nuclei without external influence.