A subspace tear was a violently destructive phenomenon which could be created as a byproduct of an isolytic burst. Attracted to the energy output of warp cores, such a tear could continue to expand until sealed.
In 2375, the Son'a created a subspace tear in the area of space known as the Briar Patch, through the detonation of an isolytic burst, which they had fired during their attempt to prevent the USS Enterprise from leaving the Patch. The Enterprise's warp core acted like a magnet to the tear, causing it to expand. The tear was sealed when Commander La Forge ejected the warp core and detonated it, successfully neutralizing the cascade. (Star Trek: Insurrection)