  We study a new source of stochastic gravitational wave background (SGWB) from
the final collapse of a network of topological defects. Typically, the final
collapse is considered negligible for generating gravitational waves (GWs) due
to its subdominance compared with the network's long-term evolution in the
scaling regime. However, in some cases, a network can be driven outside of
horizon by inflation and later re-enter horizon. Then, the network's final
collapse after re-entering horizon becomes the dominant GW source and therefore
cannot be neglected. We demonstrate this phenomenon in the context of $N_{\rm
DW}=1$ string-wall networks which naturally arise in axion models, although the
framework can be generalized to other types of topological networks. The final
collapse of walls bounded by strings releases GWs. Our calculation of the
corresponding GW spectrum suggests it could be related to the first few bins of
the nano-Hertz SGWB signal possibly detected by various Pulsar Timing Array
(PTA) collaborations. However, it is important to note that such GW spectrum
falls within a relatively narrow frequency range, which may not completely
account for the PTA signal that spans more than one order of magnitude in
frequency. Furthermore, with different parameter choices, the resultant GWs
generated in this mechanism could be probed by various GW interferometry
experiments.
