Medusa Cascade

From Tardis Wiki, the free Doctor Who reference

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.

Behind the Scenes

In an interview with DWM, Russell T. Davies claimed the Medusa Cascade would play a major part In Series 4.