The "Fleshbeast attack!" event creates burrows from which fleshbeasts spew forth. These burrows can appear under literally anything, and it will mini-fy structures that can be moved, or outright destroy those that can't.
I'm playing with the Vault mod, and one of the burrows appeared under my supply slingshot, permanently destroying it. They also destroyed some vault walls which can't be rebuilt, and caused the overhead mountain to collapse a bit, claiming one of my mechs for an instant kill.

Is it possible to make a mod that disables this event entirely? Or makes it so that burrows cannot spawn underneath structures at all? Would love that mod to bits.