The unstable rift is a rift found in the centre of the cache while playing Guthixian Cache and another unstable rift is also found in the Hall of Memories.
The players who are gathering memories are to convert them in the unstable rift to earn points in the distraction and diversion. Ancient, confused and enraged automatons will often be found scattered along the paths to the centre, trying to break the player's memory that they're carrying.
Hall of Memories
Players can deposit full memory jar in the unstable rift to obtain Divination experience. Empty memory jars can be obtained from the eastern room and then filled by siphoning memories in the Hall.