First observed directly by the Cowan-Reines neutrino experiment.
Because they interact weakly with matter and mostly just escape out of nuclear reactors, you can likely locate all nuclear reactors on Earth by measuring neutrino flows:The CIA must love that shit, they must have had it years prior to this public paper.
Articles by others on the same topic
A neutrino is an electrically neutral spin-one-half lepton with very weak ordinary interactions. The three active flavors are associated with the electron, muon and tau in weak charged current production and detection. A flavor state is generally a superposition of neutrino mass eigenstates, permitting neutrino oscillations during propagation.
Detection infers a neutrino from its interaction products rather than observing an ionizing track from a free neutrino. A charged-current event can produce the corresponding charged lepton, while a neutral-current event leaves a neutrino in the final state and deposits energy in the target. Small interaction probabilities motivate large target masses and careful control of backgrounds. Neutrality does not imply zero mass; oscillations constrain differences of squared masses, rather than fixing the absolute mass scale by themselves.
A neutrino is a subatomic particle that is part of the Standard Model of particle physics. It is electrically neutral, has an incredibly small mass (which is still not precisely measured but is known to be much less than that of an electron), and interacts very weakly with other matter, meaning it can pass through ordinary matter almost undetected.