Titanic (Donna's World)

The Titanic diverged from its original timeline on 25 June 2007, when Donna Noble turned right instead of left; this small change in affairs produced Donna's World.

Following the death of the Tenth Doctor a year earlier, a spacefaring replica of the RMS Titanic fell from the sky on 25 December 2008, crashing into Buckingham Palace. Donna Noble and her immediate family saw the falling ship on the news from the safety of Firbourne House, then felt the shockwave as it crashed. (TV: Turn Left)

While the resulting explosion wasn't enough to wipe out all life on Earth, (TV: Turn Left) as could have been the case, (TV: Voyage of the Damned) it obliterated London and resulted in a mushroom cloud which covered the whole of southern England with radiation.

Watching the cloud rise over London, Sylvia Noble mournfully assumed that everyone they knew had died.

The crash of the Titanic led to a refugee crisis, starting a series of events which resulted in the country becoming a police state. (TV: Turn Left)