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.
it is beleived that the medusa cascade is to be the story arc of the 2008 series.
in the 2008 episode The Fires of Pompeii caecillius's child who can see the future mentions the medusa cascade when commenting on the doctor.
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"