When cosmic rays collide with atoms in our upper atmosphere, showers of secondary particles are produced. This secondary radiation includes pions, which quickly decay into muons and muon neutrinos. There are three types of neutrinos: muon neutrinos, electron neutrinos, and tau neutrinos. It is theorized that neutrinos of any type change from one type to another, but scientists have yet to observe a muon neutrino changing into an electron neutrino. Making this observation for the first time is one of the goals of the NOvA experiment at Fermilab. Check out this link to see cosmic ray collisions detected by the NOvA Detectors happening in real-time, or watch the video below to see an outline of the NOvA experiment.