Flint groundwater was contaminated by the DNA of Diluvian. This DNA mutated and gave powers to anyone who ingested it. These mutants were driven insane after a short while.
The containment zone turned into the Flint Swamp after Gold Morning.
The ruins of Flint(Michigan), still saturated with mutagenic water, the mutants have seemingly evolved over eight years, forming together into large amalgamations of biomass that can be extremely destructive. The swamp is currently walled off and guarded by Flint n’ Steel, A group focused entirely on protecting the world from the horrors within the swamp.