We performed high-intensity subpicosecond laser-plasma interaction experiments to examine nonlinear scattering mechanisms in underdense plasmas. At incident laser intensities of 2 x 10(18) W/cm(2) the stimulated-Raman-backscattered spectrum exhibited an extremely broad, supercontinuumlike structure (Deltaomega/omega(0) > 1) extending from ~500 to >1200 nm (limited only by detector sensitivity). Large-amplitude modulations in the spectrum of the backscattered light were measured and are attributed to an interaction of the stimulated-Raman-scattered radiation with ion plasma waves.