Metals usually exist in form of polycrystalline solids, in which the networks of disordered grain boundaries tend to get eliminated through grain coarsening upon heating or straining, or to transform into metastable amorphous states when the grains are small enough. This is why nano-grained metals have a much reduced stability relative to their coarse-grained counterparts. Through experiments and MD simulations, we recently discovered a new metastable state for extremely fine-grained metals (typically below 10 nm), namely Schwarz crystal structure with 3D minimal interfaces constrained by twin boundaries. The polycrystalline structure is stable against grain coarsening even close to the equilibrium melting point and exhibits a hardness in vicinity of the theoretical value. In this presentation, I will introduce the formation process, structure characteristics, and some properties of the Schwarz crystal structures in a number of pure metals and alloys.