Medusa Cascade

The Medusa Cascade was a rift in time and space; at some point this was sealed by the Doctor. (DW: Last of the Time Lords) It was first visited by the Doctor when he was 90 years old. (DW: The Stolen Earth)
 * Based on the Master's familiarity with the event, he was most likely involved, possibly opening the time rift himself.

Evelina, who could see the future, mentioned the Medusa Cascade when commenting on the Doctor, saying "Your real name is hidden. It burns in the stars, in the Cascade of Medusa herself". (DW: The Fires of Pompeii)

Nearing the end of the Last Great Time War, the Doctor intended to use the The Great Key to seal the rift at the Medusa Cascade, believing that doing so could end the war. (IDW: The Forgotten)

The Doctor told Donna Noble that, amongst other places, he wanted to take her to visit the Fifteenth Broken Moon of the Medusa Cascade. (DW: The Sontaran Stratagem)

The Doctor used the words "Medusa Cascade" to see if the possessed Sky Silvestry would echo him. (DW: Midnight)

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 sync from the rest of the universe by a second. (DW: The Stolen Earth) 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 War in the Medusa Cascade. It ended with the destruction of the Daleks by the Doctor (with the help of his clone and all of his companions) and the return of all of the planets to their correct locations in time and space. The Cascade was also used to send Jackie Tyler, Rose Tyler and the Doctor's clone to Pete's World. (DW: 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. (DW: Victory of the Daleks)