The legend of the Sewer Alligator, which stretches back to the 1930's, is that of an escaped or released pet crocodile or alligator finding it's way into the sewers of a city, and thriving there. They have been sighted in various, mainly American cities, such as New york. There is controversy as to whether any still live there.
The original story is that of a family coming back from a long vacation, and bringing baby alligators back for their children to play with. Over time, the alligators would become too large and the families would dispose of them by either selling them to a nearby zoo, flushing them down the toilet, or dumping them in the sewers. The latter two of the choices is what sparked the alligators to thrive in the sewer, and they would reside in their, reproducing and growing to huge sizes, much to the shock of sewer workers.
In some versions of the story, the alligators would grow to tremendous sizes, and be a ghostly whitish color because of their lack of sunlight. Also, they would go blind because of the darkness. Another reason for albino alligators to retreat to the sewers is because they have no protection against the sun, thanks to the light pigment of their skin.