Transition of the ground state of a classical Phi^4 theory in 2+1 dimensions is studied from a metastable state into the stable equilibrium. The transition occurs in the broken Z_2 symmetry phase and is triggered by a vanishingly small amplitude homogeneous external field h. A phenomenological theory is proposed in form of an effective equation of the quantitatively accounts for the decay of the false vacuum. The large amplitude transition of the order parameter between the two minima displays characteristics reflecting dynamical aspects of the Maxwell construction.
