The Cataclysm is the end of the world, caused by the Great Breaks the SQ made. It is what the Hystorians are trying to prevent, and the reason the series takes place. In The Trap Door, Sera and Ilsa went forward in time to the Cataclysm. When they returned, Ilsa wrote a letter about it, leaving her descendant Tilda to believe that the Cataclysm is caused by the formation of the Hystorians, although the opposite is true. In Behind Enemy Lines, it is proved that humanity has survived the Cataclysm.