The Breach

The Breach, identified by the Tenth Doctor as a spatial disturbance and a "hole in the fabric of reality", was an opening to the Void which appeared on Earth in London as a "radar black spot" 600 feet above sea level. (TV: Army of Ghosts) Captain Jack Harkness knew it as the London Rift, which Torchwood One exploited whilst his Torchwood Three continued to tend to the Cardiff Rift. (PROSE: Risk Assessment)

History
The Breach was caused by what Torchwood knew as the Sphere, actually a Void Ship which was used by the Cult of Skaro, a group of four Daleks to escape the Time War, fleeing the universe into the Void along with the Genesis Ark, a Time Lord prison ship containing millions of Daleks. As the Doctor observed, it "cracked the world around it", "the entire surface of this dimension splintered." The black spot was detected by Torchwood London who, having seen warning signs for years, knew it to be a "hole in the world". Eventually, they built the skyscraper Torchwood Tower, known to the public as Canary Wharf, in order to reach it. Probing the Breach, Torchwood found it to be a massive source of energy, which they intended to exploit. In 2007, what appeared to be ghosts began appearing, "bleeding through the fault lines". Torchwood encouraged them to appear through ghost shifts which their personnel controlled through levers which could open and close the Breach, allowing Torchwood to harness the resulting energy.

Unbeknownst to Torchwood, these ghosts were actually Cybermen who were crossing over from a parallel Earth, following in the wake of the Sphere; the Doctor had encountered the Cybermen on that world earlier as a result of the TARDIS falling through the Void. After a period of about two months, the Doctor discovered the ghost shift which led him to Torchwood Tower. There, he identified the Sphere and attempted to warn Yvonne Hartman, director of Torchwood, of the potential consequences of exploiting the Void. An advance guard of Cybermen were uncovered by the Doctor, proceeding to seize control of the equipment and increase the shift to 100%, revealing all ghosts across the world to be an 5,000,000-strong army of Cybermen. At the same time, the Cult of Skaro disembarked from the Sphere.

Encountering the Preachers, who had developed technology to transport individuals across universes, the Doctor crossed with them to the parallel Torchwood, where he spoke with Pete Tyler. Pete made note of what appeared to be symptoms of global warming affecting his Earth, which he realised were being exacerbated by the Breach. The Doctor warned Pete that even the Preachers' smaller jumps were ripping holes in the universe, endangering both worlds. Returning to his Earth, the Doctor saw the opening of the Genesis Ark, unleashing the Daleks over Earth.

Knowing that the Daleks and the Cybermen were "soaked" in void stuff, the Doctor prepared to open up the Void in reverse, which would pull them into the Void; due to the Doctor's brief crossover with Rose Tyler, the two of them used Magna-Clamps to resist the pull. An attempt by the Cyber-Leader to retreat through the Breach in order to "regain the homeworld" was stopped by the now cyber-converted Yvonne Hartman. Meanwhile, Dalek Sec detected that the Breach was active and sent Daleks to exterminate the Doctor, who arrived just late as the Breach was opened. While the Cult of Skaro escaped via an emergency temporal shift, the millions of Daleks and Cybermen were pulled into the Void. When Rose was forced to raise her lever to complete the transfer, she lost her grip of the Magna Clamp and was almost pulled in herself, only to be retrieved by Pete, who was forced to take her with him as he returned to his world. The Breach then closed, leaving Rose in the parallel universe and separating her from the Doctor. (TV: Army of Ghosts/Doomsday)