The Cascade is an event in the history of the PPC, in which Makes-Things caused a chain reaction of plothole formation which spread throughout the Multiverse. It is also the term used for the plothole network created by the event.
The Cascade occurred in the mid-1980s HST. A week after his recruitment, Makes-Things activated his first upgraded portal generator. The new design created portals automatically, at a rate of one per second; the portals were self-powered, and included a degree of distributed programming which ensured they would continue to propagate even if the original generator was deactivated.
Unfortunately, Makes-Things forgot to include an off switch.
The network of plotholes expanded explosively. The Weeds, led by Captain Dandy, set out to explore the network and try to slow down its growth; meanwhile, the newly-formed Department of Building Operations and Maintenance rapidly expanded HQ into the network.
The Fall of Origin
One consequence of the creation of the Cascade was the destruction of the Flowers' homeworld of Origin. A large number of Cascade plotholes opened in the space around Origin, destabilising its orbit relative to the Hole and causing the planet to spiral inwards.
A group of dissidents on Origin rigged up a portal stabiliser, and managed to sustain a portal to one of the worlds the PPC had claimed. The Sub Rosa and Tiger Lily passed through to investigate, and began the evacuation of Origin.
The Government on Origin fought the PPC's evacuation efforts, even while earthquakes tore at the planet. In the final attack, the Government was only prevented from destroying Old HQ by the appearance of the Weeds through the Cascade.
Ending the Cascade?
A year after the Cascade was initiated, the rate of plothole creation stopped increasing. The PPC was unsure of the precise cause of the slowdown, attributing it variously to the Cascade 'simply stopping', or to Captain Dandy's intervention.
It is uncertain when, if ever, plothole creation completely stopped.
The nature of the Cascade plotholes led to HQ's connection with various areas of canon locations, such as the Shrieking Shack and a mountain range in Arda, and the network formed was used to rapidly expand Headquarters. The various far reaches of the plothole network were kept in check by HQ's shields, but the failure of those shields can allow bits of other worlds to enter HQ, which could potentially lead to a collapse of the entire complex if left unchecked. The shields have been known to have gone down twice: during 2006, which allowed the Black Cats to invade, and during 2013, when their failure was a direct result of the Blackout.
In theory, the Cascade can only lead to Word Worlds; however, it appears to have left a large number of plotholes to World One, and opened many to Origin before its fall. This suggests that the prevailing theories about the multiverse may be flawed.