  We show experimentally that a stable wave propagating into a region
characterized by an opposite current may become modulationaly unstable.
Experiments have been performed in two independent wave tank facilities; both
of them are equipped with a wavemaker and a pump for generating a current
propagating in the opposite direction with respect to the waves. The
experimental results support a recent conjecture based on a current-modified
Nonlinear Schr\"odinger equation which establishes that rogue waves can be
triggered by non-homogeneous current characterized by a negative horizontal
velocity gradient.
