Simulation of a cosmological axion through the QCD phase transition, where low-mass, ultracompact dark matter minihalos arise from density perturbations that are generated from the non-trivial axion self interactions during and shortly after the collapse of the axion-string and domain-wall network. The visualization shows two snapshots during the epoch of phase transition: the string network is shown as yellow chords, domain walls as a red mesh, and the energy density of the axion field as a blue-white intensity medium.