The Medusa Cascade was a rift in time and space; at some point it was sealed by the Doctor. (TV: Last of the Time Lords) It was first visited by the First Doctor when he was a child of ninety years old. (TV: The Stolen Earth)
Evelina, a woman who could see the future, mentioned the Medusa Cascade when commenting on the Tenth Doctor, commenting, "Your real name is hidden. It burns in the stars, in the Cascade of Medusa herself". (TV: The Fires of Pompeii) The Tenth Doctor later confided in Donna Noble that, amongst other places, he wanted to take her to visit the Fifteenth Broken Moon of the Medusa Cascade. (TV: The Sontaran Stratagem) During a solo adventure, the Doctor exclaimed "Medusa Cascade" to see if the possessed Sky Silvestry would echo him. (TV: Midnight)
The Medusa Cascade was the place where Davros's New Dalek Empire moved twenty-seven planets. The planets could not be found by regular means, as they were out of temporal synchronisation with the rest of the universe by a second. Davros placed them there so that when his reality bomb was set off, it could penetrate the Cascade's space-time rift and spread the bomb's destructive force across every possible timeline and all of existence.
This led to an invasion of Earth, one of the required twenty-seven planets. It ended with the near-total destruction of the Daleks by the Meta-Crisis Tenth Doctor and (with the help of the Doctor and his companions) the return of all of the planets to their correct locations in time and space. (TV: The Stolen Earth / Journey's End)