  The late stages of the relaxation process of a black hole are expected to
depend only on its mass and angular momentum and not on the details of its
formation process. Inspired by recent analogue gravity experiments, which
demonstrate that certain black hole processes take place in gravitational and
hydrodynamical systems alike, we conduct an experiment to search for
quasinormal mode oscillations of the free surface of a hydrodynamical vortex
flow. Our results demonstrate the occurrence and hint at the ubiquity of
quasinormal ringing in nonequilibrium analog black hole experiments.
