  Quantized magnetic vortices driven by electric current determine key
electromagnetic properties of superconductors. While the dynamic behavior of
slow vortices has been thoroughly investigated, the physics of ultrafast
vortices under strong currents remains largely unexplored. Here we use a
nanoscale scanning superconducting quantum interference device to image
vortices penetrating into a superconducting Pb film at rates of tens of GHz and
moving with velocities up to tens of km/s, which are not only much larger than
the speed of sound but also exceed the pair-breaking speed limit of
superconducting condensate. These experiments reveal formation of mesoscopic
vortex channels which undergo cascades of bifurcations as the current and
magnetic field increase. Our numerical simulations predict metamorphosis of
fast Abrikosov vortices into mixed Abrikosov-Josephson vortices at even higher
velocities. This work offers an insight into the fundamental physics of dynamic
vortex states of superconductors at high current densities, crucial for many
applications.
