The Medusa Cascade was a colourful nebular region of space and the location of a rift in time and space which was sealed at some point by the Doctor.
It was located "halfway across" the universe from Earth. (TV: The Battle of Ranskoor Av Kolos)
Appearance
The Cascade was nebulous in appearance, containing what appeared to be clouds of coloured gas. (TV: The Stolen Earth) In addition to the space-time rift at its heart, (TV: Journey's End) it also contained at least fifteen "broken moons". (TV: The Sontaran Stratagem)
History
The Medusa Cascade was visited by the First Doctor when he was "just a kid" of ninety-years-old. (TV: The Stolen Earth) He sealed the rift as part of his practical temporal technology exam. (PROSE: Report on Term's Work)
According to another account, the Eighth Doctor is the one who sealed the Mudusa Cascade with a modified De-mat Gun. (COMIC: The Forgotten
The Fourth Doctor and Romana later visited the Medusa Cascade. (PROSE: City of Death)
In one potential timeline of the Last Great Time War, Livia and Narvin saw the Medusa Cascade burn to nothing as a result of the War Master becoming creator of the Daleks. (AUDIO: The Master's Dalek Plan)
The Medusa Cascade was the place to which 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 one second. (TV: The Stolen Earth) 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. (TV: Journey's End)
This act led to an invasion of Earth, one of the required twenty-seven planets. (TV: The Stolen Earth) 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: Journey's End)
References
Evelina, a woman who could see the future, mentioned the Medusa Cascade when commenting on the Tenth Doctor, stating, "Your real name is hidden. It burns in the stars, in the Cascade of Medusa herself". (TV: The Fire 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 Sontaran Subliminal Education Matrix mentioned the Medusa Cascade to Strax as a location home to alien enemies of the Sontaran Empire. (PROSE: A Soldier's Education)