The Great Bloom (or Riftmouth to the Neyel) was one end of a tunnel through space-time that connected the Milky Way Galaxy's Beta Quadrant with the Small Magellanic Cloud. The Milky Way side was located inside the Bassen Rift, close to Romulan space.
It was formed there in late 2379 when the Reman warbird Scimitar, under the command of Shinzon, was destroyed in battle with the USS Enterprise-E, as well as the Romulan warbirds IRW Valdore, under the command of Commander Donatra, and IRW Soterus, under the command of Commander Suran.
Early in the year 2380, after the recent death of Shinzon and the resulting political fallout, Donatra and Suran chose to hide close to the Bloom a large portion of a fleet of warbirds loyal to them. When Donatra, aided by the USS Titan, came to investigate the loss of contact with her fleet, the exact nature of the Bloom was eventually discovered, along with the fact that it allow a protouniverse, dubbed the "Sleeper" or "Red King", to emerge into the primary universe. Soon afterward, the rift was closed through the ejection and detonation of warp cores of more than two dozen Romulan warbirds. (TTN novel: The Red King)