The Swamp is a location featured in RAGE. Best known as the home of mutated plants and Giant Flies, it is found not far from the Mutant Bash TV studio. Crazy Joe lives here and states that the mutated plantlife around can be used to make "special things", like the Apophis Infusion.
Take the left path from Wellspring. Head up the highway ramp and drive to the end, past Mutant Bash TV. Head towards the Authority gate, and by now, you should see some blue on your minimap. That's where it is.