In ferromagnetic nanostructures domain walls as emergent entities separate uniformly magnetized regions. They are describable as quasi particles and can be controlled by magnetic fields or spin-polarized currents. Below critical driving forces domain walls are rigid conserving their spin structure. Like other quasi particles internal excitations influence the domain wall dynamics above a critical velocity known as the Walker breakdown. This complex nonlinear motion has not been observed directly. Here we present direct time-resolved x-ray microscopy of structural transformations of domain walls during motion. Although governed by nonlinear dynamics the displacement of the wall on the observed time scale can still be described by an analytical model. Using a reduced dynamical domain-wall width the model enables us to determine the mass of a vortex wall experimentally. Further we observe the creation and the mutual annihilation of domain walls. The intrinsic nanometer length and nanosecond time-scales are determined directly.