  In this contribution, we discuss the cosmological scenario where unstable
domain walls are formed in the early universe and their late-time annihilation
produces a significant amount of gravitational waves. After describing
cosmological constraints on long-lived domain walls, we estimate the typical
amplitude and frequency of gravitational waves observed today. We also review
possible extensions of the standard model of particle physics that predict the
formation of unstable domain walls and can be probed by observation of relic
gravitational waves. It is shown that recent results of pulser timing arrays
and direct detection experiments partially exclude the relevant parameter
space, and that a much wider parameter space can be covered by the next
generation of gravitational wave observatories.
