(covers information from several alternate timelines)
The Hekaras Corridor was a narrow region of space twelve light years long that passes through the Hekaras system. There were intense tetryon fields around the Corridor, making it the only safe way to cross the region at warp.
In the 2360s, Hekaran scientists Rabal and Serova attempted to convince the Federation Science Council that cumulative exposure to warp fields was destabilizing the Corridor, which would eventually render their homeworld of Hekaras II uninhabitable. Unable to convince the Council to take prompt action, the two took to placing buoys rigged to produce verteron pulses inside the Corridor to call attention to their findings. In 2370, these buoys disabled the USS Fleming, a Ferengi marauder, and the USS Enterprise-D.
Eventually, Serova caused a warp core breach aboard her own vessel to prove her theory that exposure to warp field energy would cause a subspace rift. Afterward, the Federation Council imposed a warp 5 speed limit on all vessels except in extreme emergencies, while limiting warp travel in vulnerable areas, such as the Corridor, to essential traffic only. (TNG: "Force of Nature")