The minimum energy pathway (MEP) for H atom addition to H2O has been determined by ab initio calculations at the MP3 level using a large Rydberg basis set. In agreement with previous theoretical studies, a local minimum in a C3v geometry was found lying ∼ 18.4 kcal/mol above the energy of H2O + H, but separated from the products by a 3.9 kcal/mol barrier. The well depth is sufficient to contain a quasi-bound zero point level; however, tunneling lifetimes, estimated to be 10-13-10-12 s, contrast sharply with the experimental results of neutralized ion beam studies, which indicate a ∼ 1-μs lifetime for D3O. The potential energy surface (PES) near the MEP was determined over a region sufficient to allow the PES to be approximated by a harmonic hypervalley around the MEP. Application of a vibrationally adiabatic approximation to the discussion dynamics indicates that certain vibrational levels may exhibit significantly enhanced lifetimes. The type and extent of vibrational excitation minimally required to account for the experimental lifetimes is discussed.