Doctor Who: 20 Things You Didn’t Know About The Waters Of Mars
3. The Cloister Bell Returns
The Waters of Mars takes quite a dark turn at the end, after Adelaide Brooke kills herself in order to keep her timeline intact.
This essentially spells the end for the Tenth Doctor, as his flagrant abuse of time travel law curses him to meet his maker. His impending doom is heralded by the arrival of Ood Sigma, and a sound that must've sent chills down the Time Lord's spine. And everyone's spine, come to think of it.
That sound is from the Cloister Bell, essentially a warning system inside the TARDIS.
Though the bell had been used in NuWho before (it can be heard in The Sound of Drums and Turn Left, for example), this was the first time since 1981's Logopolis that it had been used to signal the Doctor's death.