  We develop a new method for estimating the decay probability of the false
vacuum via regularized instantons. Namely, we consider the case where the
potential is either unbounded from below or the second minimum corresponding to
the true vacuum has a depth exceeding the height of the potential barrier. In
this case, the materialized bubbles dominating the vacuum decay naturally have
a thick wall and the thin-wall approximation is not applicable. We prove that
in such a case the main contribution to the action determining the decay
probability comes from the part of the solution for which the potential term in
the equation for instantons can be neglected compared to the friction term. We
show that the developed approximation exactly reproduces the leading order
results for the few known exactly solvable potentials. The proposed method is
applied to generic scalar field potentials in an arbitrary number of
dimensions.
