Each of the 52 universes are surrounded by a Source Wall, with the Bleed between the source walls of the various universes. The Monitors refer to an impending disaster if they don't act to stop travel between the 52 universes, and reveal that each time a being passes between universes, they break a little of each universes' Source Wall. If a source wall is breached too many times, it will collapse and lead to the chaos and destruction of its universe.
