The sun was 'missing' two-thirds of its neutrinos for over 30 years

Starting in the 1960s, Ray Davis's underground tank of cleaning fluid detected only a third of the solar neutrinos nuclear physics predicted. The shortfall wasn't a flaw in the sun or the detector — electron neutrinos were switching into other 'flavors' mid-flight. The Sudbury Neutrino Observatory confirmed this oscillation in 2001, solving a decades-old mystery and proving neutrinos have mass.

— Q. R. Ahmad et al. (SNO Collaboration), Measurement of the Rate of Interactions Produced by 8B Solar Neutrinos at the Sudbury Neutrino Observatory — Physical Review Letters, 2001