Barsoom

Sixth Doctor has a map of Barsoom  in a TARDIS storage room. One place was labled Helium and the name John Carter was included in the caption. (COMIC: Changes)

Behind the scenes
Barsoom was the name for Mars in the Martian Series of novels by Edgar Rice Burroughs. John Carter was the protagonist of the series and Helium was a major kingdom of Barsoom.