We report the experimental observation of a novel transmission phenomenon in optical long-haul communication systems. Unpolarized ASE depolarizes via nonlinear fiber interactions a cw laser light during their copropagation which leads to small but measurable ultrafast polarization state fluctuations at the fiber output. We provide a phenomenological approach and a theory that qualitatively corroborates our experimental results. One of our major findings suggests that the applicability of the often used Manakov equation needs to be scrutinized for highly accurate studies of nonlinear polarization state evolutions in noisy environments. The described phenomenon leads to a qualitatively different microscopic understanding of nonlinear light propagation in fiber and can contribute toward an explanation for today's commonly perceived gap between simulated and experimentally obtained system performance in optical data transmission.