  Graphene hosts a unique electron system in which electron-phonon scattering
is extremely weak but electron-electron collisions are sufficiently frequent to
provide local equilibrium above liquid nitrogen temperature. Under these
conditions, electrons can behave as a viscous liquid and exhibit hydrodynamic
phenomena similar to classical liquids. Here we report strong evidence for this
transport regime. We find that doped graphene exhibits an anomalous (negative)
voltage drop near current injection contacts, which is attributed to the
formation of submicrometer-size whirlpools in the electron flow. The viscosity
of graphene's electron liquid is found to be ~0.1 m$^2$ /s, an order of
magnitude larger than that of honey, in agreement with many-body theory. Our
work shows a possibility to study electron hydrodynamics using high quality
graphene.
