A number of experimental and theoretical studies have been carried out on the anionic water hexamer in the last decade. However, none of these studies have reported the adiabatic electron detachment energy. The present study employing extensive high-level ab initio calculations report the adiabatic electron detachment energy, which explains the unusual stability of the anionic water hexamer. This stability can be correlated to the unusually intense peak observed in the photoelectron-detachment spectra. It is also shown that our previously predicted pyramid structure reproduces the important characteristics of the experimental O-H vibrational spectra.