A subspace inversion is an event that takes place in the Bajoran wormhole every fifty years. It can also be created artificially.
It has been theorized that the subspace inversions that occur every fifty years have been visible from Dahkur Province on Bajor for at least three and half thousand years. (DS9 reference: Star Trek: Deep Space Nine Technical Manual)
In 2372, the USS Defiant observed a natural subspace inversion in the wormhole. The Defiant was damaged and Sisko repaired damage to the warp core, narrowly missing an electric discharge from the core.
In an alternate timeline, Sisko was hit by the discharge and shifted out of time. Fifty years later, Jake Sisko, Nog, Julian Bashir and Jadzia Dax attempted to rescue Sisko during a subspace inversion, although the attempt failed. (DS9 episode: "The Visitor")