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 Doctor when he was a child of ninety years old. (TV: The Stolen Earth)
Evelina, who could see the future, mentioned the Medusa Cascade when commenting on the Tenth Doctor, saying "Your real name is hidden. It burns in the stars, in the Cascade of Medusa herself". (TV: The Fires of Pompeii)
The Medusa Cascade was also the place where Davros's New Dalek Empire moved 27 planets. The planets could not be found by regular means, as they were out of 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 space-time rift in the Cascade and spread the bomb's destructive force across every universe and all of existence.
This led to the invasion of Earth, one of the 27 planets. It ended with the destruction of the Daleks by the Meta-Crisis Tenth Doctor and (with the help of the Doctor and all of his companions) the return of all of the planets to their correct locations in time and space. (TV: The Stolen Earth / Journey's End) Although it seemed that all the Daleks had been destroyed, one ship escaped through time to World War II. (TV: Victory of the Daleks)