  After reviewing the shortcomings of existing definitions used to characterize
the boundary of a black hole, we present a new method for its characterization.
This definition could potentially be applied to locate the boundary of general
black hole solutions of the Einstein field equations. Using our definition in
general spherically symmetric spacetimes, we argue that an observer falling
into a black hole encounters an infinite energy density, pressure, and flux at
the black hole boundary. This result is interpreted as a firewall that prevents
the growth of classical black holes. However, the possibility of black hole
growth via quantum mechanical tunneling cannot be ruled out.
