Strepto

Strepto was the home planet of the alien Water Hags. Its landscape was mainly comprised of a marsh-like, boggy terrain. At some point during the Middle Ages, it was apparently lost by the Water Hags, who claimed it vanished into thin air. (TWN: Something in the Water)

In fact, the planet had been stolen by the Daleks and was one of the 27 planets they needed to activate the reality bomb. It was returned to its rightful place and time. (REF: The Time Traveller's Almanac)

''Strepto was stolen during the Middle Ages, but 24 planets were taken in 2009, and the other three, "the Lost Moon of Poosh", Pyrovillia and Adipose 3 were seemingly the only ones taken out of time. This suggests that Strepto was an extra 28th planet which the Tenth Doctor didn't know of.''