The sewers of both New New York and Old New York are a maze of pipes and plumbing that are impossible to travel through without a guide. Unfortunately, the only guides available are the mutated humans far below the streets of Old New York. They live here in a society composed of crumbling fecal matter, and other refuse from the civilisation above.
They worship an unexploded nuclear warhead, although not many are regular churchgoers, and live in fear of El Chupanibre and other such monstrosities.
The residents of this bizzare society have tamed the crocodiles and alligators that were flushed down the toilets of a million Americans, and also keep goldfish as pets. They use toilet paper for anything possible, even to the extent of holding tissue tape parades to honour their heroes.
Leela's parents live among the mutants, too.