Planar ordered honeycomb-like structures of the MnO₂ layered birnessite-type crystalline form have for the first time been synthesized on the surface of Mn(OAc)₂ solution drops upon interaction with O₃ gas. The obtained network consists of 2D nanocrystals of the thickness up to 10 nm. The prepared material possesses a hierarchical structure and can be used in a fabrication of advanced sensor and electrode materials.