The ablation front Rayleigh-Taylor evolution of a well-characterized 3D multimode surface perturbation was measured on a foil accelerated with an x-ray drive as it progressed into the weakly nonlinear regime. The perturbation pattern was locally random and consisted of modes within the first ten harmonics of a 300 {mu}m square. The pattern evolved into bubbles separated by narrow, interconnecting spike sheets, unambiguously indicating entry into the nonlinear regime. Nonlinear effects occurred when mode amplitudes were relatively small: {eta}{sub n}{lt}{lambda}{sub n}/10 . Experimental results are in quantitative agreement with results of our numerical simulations. {copyright} {ital 1998} {ital The American Physical Society}