  We consider the simplest model which solves the solar and atmospheric
neutrino puzzles, in the sense that it contains the smallest amount of beyond
the Standard Model ingredients. The solar neutrino data is accounted for by
Planck-mass effects while the atmospheric neutrino anomaly is due to the
existence of a single right-handed neutrino at an intermediate mass scale
between 10^9 GeV and 10^14 GeV. Even though the neutrino mixing angles are not
exactly predicted, they can be naturally large, which agrees well with the
current experimental situation. Furthermore, the amount of lepton asymmetry
produced in the early universe by the decay of the right-handed neutrino is
very predictive and may be enough to explain the current baryon-to-photon ratio
if the right-handed neutrinos are produced out of thermal equilibrium. One
definitive test for the model is the search for anomalous seasonal effects at
Borexino.
