Thames Flood Barrier

The Thames Flood Barrier was a flood control structure on the River Thames.

On 24 December 2007 it was partially destroyed by the Tenth Doctor. He used explosives to blow up strategic points of the structure. This let the Thames flood the abandoned Torchwood Institute laboratory underneath and flow to a hole in the middle of the secret base. It extended all the way to the centre the Earth. This drowned the children of the Racnoss, who were emerging from this hole. The Thames remained closed for over a year, resulting in great financial problems for Britain. (TV: The Runaway Bride)

In an alternate timeline, while the Doctor still killed the Racnoss's children, he also perished in the flooding (perhaps even letting himself be killed). This led to a chain of events that drastically altered history. The Doctor's former companion, Rose Tyler, along with the alternate timeline version of Donna Noble, eventually restored the original timeline. (TV: Turn Left)