A subspace rupture was a phenomenon which could allow for passage to a subspace layer. Creating a stable warp field was impossible in the vicinity of a subspace rupture, and, if large enough, a rupture could be extremely destructive, even having the potential to destroy an entire star system. (DS9: "If Wishes Were Horses"; VOY: "Cold Fire", "The Omega Directive")
Encounters
In the mid-23rd century, rupture was reported in the Hanoli system by a Vulcan science mission. In an attempt to seal the rupture, the Vulcans detonated a pulse-wave torpedo at the coordinates of the tear. Their plan backfired, however, and the entire Hanoli system was destroyed. Shortly thereafter, the rupture reached critical mass and imploded, disappearing from normal space altogether. (DS9: "If Wishes Were Horses")
In 2369, the crew of Deep Space 9 encountered what they thought to be a subspace rupture in the Bajoran system. Attempting to improve upon the intentions of the Vulcan crew, they also detonated a pulse-wave torpedo in the rupture, but the effort proved futile. The crew eventually realized that the rupture was not real, but rather a manifestation of what Lieutenant Jadzia Dax thought to be a subspace rupture, due to their interaction with a race of aliens who had the ability to bring fantasy into reality. When Dax realized that it was only her imagination which was imbuing the rupture with reality, the rupture disappeared. (DS9: "If Wishes Were Horses")
The Nacene known only as "Suspiria" gained access to the normal spacetime continuum from her subspace realm of Exosia via subspace rupture. In 2372, the USS Voyager encountered Suspiria in the Delta Quadrant, and was nearly destroyed by the powerful being, but Captain Janeway was able to counter her attack with a sporocystian toxin, forcing her to retreat back through the rupture. (VOY: "Cold Fire")
The creation of subspace ruptures was a known side effect of the detonation of Omega molecules. In the 23rd century, such a detonation made warp travel impossible in the Lantaru sector due to ruptures extending several light years from the site of the detonation. In 2374, Voyager encountered an identical phenomenon in the Delta Quadrant, when it was discovered that a species there was attempting to use Omega to power their civilization. (VOY: "The Omega Directive")