The Bassen Rift was a region of space between Romulus and the Romulan Neutral Zone composed of extremely dense areas of stellar gas. 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 reached the the Rift to conduct an ambush, as it would be impossible for the Enterprise crew to contact Starfleet to request reinforcements. (Star Trek Nemesis)