Whether neutrinos are Majorana particles—that is, whether neutrinos are their own antiparticles—remains one of the most fundamental open questions in particle physics. Scientists worldwide are searching deep underground for the extremely rare process known as neutrinoless double-beta decay (0νββ), which would provide a crucial test of the Majorana nature of neutrinos. To reliably identify 0νββ, however, it is first essential to precisely measure the spectral shape and half-life of the Standard Model-allowed two-neutrino double-beta decay process (2νββ).