A subspace rift is a rupture of a region of subspace instability where subspace extrudes into normal space due to excessive exposure to warp field energy. Such a phenomenon is characterized by tetryon radiation and high-energy distortion waves, and is extremely sensitive to the exposure of further warp field energy.
In 2370, Hekaran scientists Serova and Rabal began mining the Hekaras Corridor with verteron probes to disable warp-powered vessels, in an attempt to gain the attention of the Federation and press their case that cumulative damage being done in that area of space was causing severe ecological damage to their homeworld. When the USS Enterprise arrived to investigate, Serova and Rabal presented their case, but were rebuffed when it appeared that they did not have enough hard evidence. In response, Serova initiated a warp core breach on her vessel, exposing a region of subspace instability to a massive amount of warp field energy to create a rift, sacrificing her life to prove her theory. Unfortunately, the rift enveloped the medical transport vessel USS Fleming, stranding them inside. The crew of the Enterprise was forced to develop a plan whereby they could rescue the Fleming without active use of their warp engines, as exposure to further warp field energy would cause the rift to expand even farther. Lieutenant Commander Data came up with the idea of initiating a high-intensity warp pulse to "coast" into the rift. While the Enterprise was inside the rift, the Fleming attempted to initiate their warp drive, an action which caused the rift to expand. After beaming the crew of the Fleming aboard the Enterprise, they were able to escape the rift by phase-matching their deflector shields to the EM variance of the distortion waves, riding the wave out of the rift. (TNG: "Force of Nature")