The Bassen Rift is a region of space between Romulus and the Romulan Neutral Zone containing interstellar gas that produced electromagnetic distortion. The Rift interfered with all long-range communications, making contact with outside vessels impossible.
In 2379 the Rift was the site of a major conflict between the USS Enterprise and the Reman warbird Scimitar, which was under the command of Praetor Shinzon. Having pursued the Enterprise from Romulus while under cloak, Shinzon chose to wait until the Enterprise entered the Rift to conduct an ambush, as it would be impossible for the Enterprise crew to contact Starfleet to request reinforcements. However, with assistance from two Valdore-type warbirds, the Enterprise managed to destroy the Scimitar. (Star Trek Nemesis)
In Star Trek Online, the Bassen Rift is located in Tarod sector (β) and called by its Romulan name Great Bloom. The description of the sector states that even thirty years after the Battle in the Bassen Rift, the region continues to be irradiated with trace amounts of thalaron radiation left over from the Scimitar's destruction.