This chapter tells the story of several decades of experimental physics done deep underground with huge detectors. Not for the first time in the history of science, the most exciting signals were not the ones originally sought. Large underground detectors were originally designed and built to search for the decay of the proton, which was predicted by Grand Unified Theories (GUTs). Later, these same detectors studied solar, supernova, and atmospheric neutrinos. The unambiguous discovery of neutrino mass and oscillations was a highlight of the end of last century; the solution to the solar neutrino problem was a highlight of the beginning of this century. Current-generation detectors acting as targets for reactor and beam neutrinos have been steadily improving our understanding of neutrinos. And the story is not over | the original quest for proton decay has not yet been fulfilled, neutrinos have not yet yielded all their secrets, and more surprises may be in store.