  Four-dimensional, asymptotically flat spacetimes with an ergoregion but no
horizon have been shown to be linearly unstable against a
superradiant-triggered mechanism. This result has wide implications in the
search for astrophysically viable alternatives to black holes, but also in the
understanding of black holes and Hawking evaporation. Here we investigate this
instability in detail for a particular setup which can be realized in the
laboratory: the {\it hydrodynamic vortex}, an effective geometry for sound
waves, with ergoregion and without an event horizon.
