Nucleon decays are generic predictions of many well motivated theories, including those based on the unification of forces and supersymmetry. I will discuss how nonstandard nucleon decays offer a unique opportunity to broadly probe light new particles beyond the Standard Model with masses below few GeV, including axion-like particles, dark photons, sterile neutrinos, and scalar dark matter. Conventional searches can misinterpret and even completely miss such new physics. I will discuss a general strategy based on momenta of visible decay final states to probe these processes, offering a rich physics program for existing and upcoming experiments such as Super-Kamiokande, Hyper-Kamiokande, DUNE, and JUNO.