  We investigate the semi-classical instability of vacuum domain walls to
processes where the domain walls decay by the formation of closed string loop
boundaries on their worldvolumes. Intuitively, a wall which is initially
spherical may `pop', so that a hole corresponding to a string boundary
component on the wall, may form. We find instantons, and calculate the rates,
for such processes. We show that after puncture, the hole grows exponentially
at the same rate that the wall expands. It follows that the wall is never
completely thermalized by a single expanding hole; at arbitrarily late times
there is still a large, thin shell of matter which may drive an exponential
expansion of the universe. We also study the situation where the wall is
subjected to multiple punctures. We find that in order to completely annihilate
the wall by this process, at least four string loops must be nucleated. We
argue that this process may be relevant in certain brane-world scenarios, where
the universe itself is a domain wall.
