  We study the effects of the environment at zero temperature on tunneling in
an open system described by a static double-well potential. We show that the
evolution of the system in an initial Schrodinger cat state, can be summarized
in terms of three main physical phenomena, namely decoherence, quantum
tunneling and noise-induced activation. Using large-scale numerical
simulations, we obtain a detailed picture of the main stages of the evolution
and of the relevant dynamical processes
