As the Tenth Doctor recalled, the Medusa Cascade was the centre of a rift in time and space.
The Doctor himself had visited the Medusa Cascade as "a kid" at the age of 90 years. (TV: The Stolen Earth) According to one account, it was on this occasion that the Doctor sealed the rift as part of his practical temporal technology exam. (PROSE: Report on Term's Work)
As the Saxon Master recalled, the Doctor had sealed the rift at the Medusa Cascade single-handedly; (TV: Last of the Time Lords) according to the Master's diary, the Time Lords had tasked the War Doctor with this during the Last Great Time War whilst the War Master was sent to stop the Dalek Emperor from taking control of the Cruciform. (PROSE: The Secret Diary of the Master) By one account, the Eighth Doctor was the one who sealed the Medusa Cascade, with a modified De-mat Gun. (COMIC: The Forgotten)
In the Planetary Relocation Incident, the New Dalek Empire took 27 stolen planets out of space-time into the Medusa Cascade, where they were to function as a transmitter for the reality bomb's wavelength which, upon detonation, would have broken through the rift at the heart of the Medusa Cascade and thus dissolved all matter across "every dimension, every parallel, every single corner of creation", leaving the Daleks within the Medusa Cascade as the only life forms in existence. Ultimately, however, the detonation was prevented by the DoctorDonna. (TV: Journey's End)