Medusa Cascade

The Medusa Cascade was a location where there was a time rift. At some point, the Doctor closed the rift in time and space. (DW: Last of the Time Lords)

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, mentions 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)

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 repeat him. (DW: Midnight)

In Journey's End, The Doctor gets trapped in the Medusa Cascade. It is unknown if he is trapped by Davros or by the Shadow Proclamation.

Behind the Scenes

In an interview with DWM, Russell T. Davies claimed the Medusa Cascade would play a major part In Series 4 and "come back to haunt us".