Ninth Dimension

The Ninth Dimension was a reality filled with many stars and planets. The Second Doctor sent the Arcturians' fleet of spaceships into the Ninth Dimension so they could find a planet on which to settle instead of Earth, as he knew that the people of Earth would most likely exterminate them. (PROSE: Only a Matter of Time)

While still feeding her mistaken belief that he was a human surgeon rather than a new regeneration of the Doctor, the Twelfth Doctor mentioned "battling giant robot fish from the Ninth Dimension" to River Song as something that the actual Doctor might be busy doing. (TV: The Husbands of River Song)