Studying the electronic properties of graphene under extreme nonequilibrium conditions has provided a productive testbed to probe and monitor exotic transport phenomena. Andersen et al. report measurements of electron transport in ultraclean graphene devices where the electron drift velocity is extremely high. They found that direct current at high drift velocities generates a large increase in the noise at gigahertz frequencies and that the noise grows exponentially in the direction of the current. The authors attribute the emission mechanism to amplification of acoustic phonons through the Cerenkov effect.Science, this issue p. 154Understanding and controlling nonequilibrium electronic phenomena is an outstanding challenge in science and engineering. By electrically driving ultraclean graphene devices out of equilibrium, we observe an instability that is manifested as substantially enhanced current fluctuations and suppressed conductivity at microwave frequencies. Spatial mapping of the nonequilibrium current fluctuations using nanoscale magnetic field sensors reveals that the fluctuations grow exponentially along the direction of carrier flow. Our observations, including the dependence on density and temperature, are consistently explained by the emergence of an electron-phonon Cerenkov instability at supersonic drift velocities. These results offer the opportunity for tunable terahertz generation and active phononic devices based on two-dimensional materials.