Medusa Cascade rift

As the Tenth Doctor recalled, the Medusa Cascade was the center of a rift in time and space. He himself had visited the Medusa Cascade as "a kid" at the age of 90 years. (TV: The Stolen Earth)

As the Saxon Master recalled, the Doctor had sealed the rift at the Medusa Cascade single-handed; (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)

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)