Stable microbubbles are a ubiquitous feature of natural waters, acting as sites for mechanical and acoustical cavitation and nucleii for bubble growth at low gas super-saturations. Stable microbubble populations that have been determined acoustically in the past are not well explained by existing models of bubble stabilization, i.e., stabilization in particle crevices or stabilization by molecular monolayers. Based upon the observation that bubbles rising in sea water rapidly become coated with particles, a model is developed in which bubbles are stabilized mechanically by monolayers of adsorbed nonpolar particles. Long-term bubble stability is demonstrated in laboratory experiments. Predictions based on the experiments and the model are, in order of magnitude, consistent with observation.