Abstract: The structure and process for making a hollow article in which the core structure is formed from two cell sheets diffusion bonded to a rib pattern sheet having a preselected pattern of ribs having passageways. The cell sheets are diffusion bonded to the rib pattern sheet using tooling pattern sheets having a geometry matching that of the rib pattern sheet thereby creating contiguous cells, interconnected by the passageways, between the cell sheets at each rib and the periphery of the structure. Inert gas is then applied to the cells during a superplastic forming process to create expanded envelopes of each cell of the core. Face sheets of the sandwich structure may then be formed from the cell sheets themselves by further expanding the cell sheets out against a mold surface. Alternatively, the envelopes of the core may be expanded and simultaneously or subsequently diffusion bonded to separate face sheets.