Silent Pool

The Silent Pool was a lake near Guildford in Surrey, a short drive from Eddison Manor. On 8 December, 1926, Agatha Christie, the Tenth Doctor and Donna Noble drowned the Vespiform Reverend Arnold Golightly in this lake. The next day, Agatha's car was found by the side of the lake by local residents. (TV: The Unicorn and the Wasp)

Behind the scenes
The name of the lake is not mentioned in-story, but its name is on a sign.