  We re-introduce, in light of our modern understanding of neutrinos, the
concept of ``macroscopic neutrinoless double beta decay" (MDBD) for Majorana
neutrinos. In this process an antineutrino produced by a nucleus undergoing
beta decay, $X\to Y + e^- + \bar \nu_e$, is absorbed as a neutrino by another
identical $X$ nucleus via the inverse beta decay reaction, $\nu_e + X \to
e^-+Y$. The distinct signature of MDBD is that the total kinetic energy of the
two electrons equals twice the endpoint energy of single beta decay. The
amplitude for MDBD, a coherent sum over the contribution of different mass
states of the intermediate neutrinos, reflects quantum coherence over
macroscopic distances, and is a new macroscopic quantum effect. We evaluate the
rate of MDBD for a macroscopic sample of ``$X$" material, e.g., tritium, acting
both as the source and the target. The accidental background for MDBD
originating from two separate single beta decays, which contains two final
state neutrinos, can be readily rejected by measuring the energy of the
detected two electrons. We discuss the similarities and differences between the
MDBD and conventional neutrinoless double beta decay.
