A 2 × 2 layer-buckled manganese oxide, K(0.66)Mn(2)O(4)·0.28H(2)O (I), has been synthesized under high pressure and retained at ambient pressure; it is metastable and will finally transform to a 2 × 1 layer-buckled K(0.99)Mn(3)O(6)·1.25H(2)O (II) in 1 year. Both crystal structures were determined by single-crystal X-ray diffraction. On the basis of these buckled layers, which are a result of ordering of Mn(3+)/Mn(4+) in separate rows and cooperative Jahn-Teller distortion of Mn(3+)O(6) octahedra, a mechanism of structure transformation from birnessite to tunnel structures was proposed.