There is a venerable urban legend of alligators living in the sewers of major metropolitan areas. It began as one baby gator stolen from the pet store by a capricious child, and once discovered by his parents, is flushed down the toilet. It grew up, spawned and proceeded to stalk the sewers for the rest of its days. Three things are patently untrue about this urban legend:
1) Alligators aren't sold in most pet stores. It wasn't stolen from one.
2) The child flushed it not because of parents, but because children as sadistic and evil monsters.
3) It was actually a baby crocodile, but most people don't know the difference.
Some version of the gator/croc infestation is real in most cities where children have access to wildlife and toilets. However the average sewer croc is friendly to the point of being entrepreneurial, perpetually stalking the sewers in order to collect all the change people drop down grates, pipes and sinks. The reason there is not one, but many crocs in the average sewer is because the original croc made enough money to get his family across the border and into the neighborhood.