A researcher stands over an underground neutrino detector, essentially a giant tank filled with the chemical perchloroethylene, deep underground in South Dakota's Homestake Gold Mine. The Homestake experiment, as it came to be known, revealed an important discrepancy between theoretical predictions regarding neutrinos and actual data; the resolution of this problem led to the discovery of neutrino oscillation, a quantum mechanical phenomenon which is not part of the original Standard Model of particle physics.