  We demonstrate experimentally multi-bound-soliton solutions of the Nonlinear
Schr\"odinger equation (NLS) in the context of surface gravity waves. In
particular, the Satsuma-Yajima N-soliton solution with N=2,3,4 is investigated
in detail. Such solutions, also known as breathers on zero background, lead to
periodic self-focussing in the wave group dynamics, and the consequent
generation of a steep localized carrier wave underneath the group envelope. Our
experimental results are compared with predictions from the NLS for low
steepness initial conditions where wave-breaking does not occur, with very good
agreement. We also show the first detailed experimental study of irreversible
massive spectral broadening of the water wave spectrum, which we refer to by
analogy with optics as the first controlled observation of hydrodynamic
supercontinuum a process which is shown to be associated with the fission of
the initial multi-soliton bound state into individual fundamental solitons
similar to what has been observe in optics.
