Role in the series
Monstropolis is the main setting of Spongebob and Friends join Monsters Inc, in which it's origins is clarified. Starting out as unscary stubby monsters amongst the prejudice of humans millions of years ago in prehistoric times evolving from secret strains of preserved life in a cave of organic primordial gene oceans, they went to an enchanted island on another world to avoid being killed or segregated. There they found a magic fruit that made them scarier, and discovered a way to harness power through scaring people thousands of years later, and used their island home's magic and shaped it with technology to create magic doors to come to the human world to scare humans, mostly children, to gather their scream energy. Though the founders of the Child Detection Agency, the families of Waternoose and Randall, and so many other monsters, spread lies about human children being overly toxic because they felt that humans didn't deserve to have continued contact with monsters just to be scared for energy, and should be avoided outright, keeping their rule over Monstropolis secure. But thanks to the Lodgers and Boo, they find harmony for good.