  The breaking of an approximate discrete symmetry, the final stages of a first
order phase transition, or a post-inflationary biased probability distribution
for scalar fields are possible cosmological scenarios characterized by the
presence of unstable domain wall networks. Combining analytical and numerical
techniques, we show that the non-spherical collapse of these domains can be a
powerful source of gravitational waves. We compute their contribution to the
stochastic background of gravitational radiation and explore their
observability by present and future gravitational wave detectors.
