Jan Harold Brunvand, in his famous book on urban legends, The Vanishing Hitchhiker (1981), debunks most of the teenagey scary-story myths on which he reports. One of the most bizarre ones tho, about alligators in the New York City sewers, he finds corroborative evidence proving true. The standard urban legend was that kids all over the city had bought baby alligators (imported for sale or brought back from vacation from Florida), &, when they grew too big or dangerous, flushed them down the toilets. Whereupon, they grew to giant, blind, sometimes albino, rat-eating, secret monsters (the story, like all urban legends, has many variants). Thomas Pynchon - another reclusive blind, albino monster - elaborates in an episode from his novel V (1963):
Did he remember the baby alligators? Last year, or maybe the year before, kids all over Nueva York bought these alligators for pets. Macy's was selling them for fifty cents; every child, it seemed, had to have one. But soon the children grew bored with them. Some set them loose in the streets, but most flushed them down the toilets. And these had grown & reproduced, had few of rats & sewage, so that now they moved big, blind, albino, all over the sewer system. Down there, God knew how many there were. Some had turned cannibal because in their neighborhood the rats had all been eaten, or had fled in terror.
There is also the Alligator Patrol, which hunts them down. Brunvand then describes 1960's legends about a potent "White Weed" which grows "lushly in the sewer because of the nutrients," (pg. 92), but people are deterred from harvesting because of the alligators. As I said, he dismisses as having no origin most of the legends he describes in the book. But this one, remarkably, can be linked back to newspaper stories from the 1930s (this headline from the New York Times, February 10th, 1935):
ALLIGATOR FOUND IN UPTOWN SEWER
Youths Shoveling Snow Into Manhole
See the Animal Churning in Icy Water
SNARE IT AND DRAG IT OUT
Reptile Slain by Rescuers
When It Gets Vicious-
Whence It Came Is Mystery
The article continues with the full story. Furthermore, there are public works records corroborating the existence of alligators:
According to a former New York City Commissioner of Sewers there was a problem with alligators in the sewers in the mid 1930s. In his book on the development of utilities beneath Manhattan Island, titled The World Beneath the City, Robert Daley claims that recurrent reports of alligators in the sewers finally forced Sewer Commissioner Teddy May to investigate the situation personally. May told Daley that he did find alligators (averaging two-feet long); he immediately launched a campaign to eradicate them [like Pynchon's Alligator Patrol!], & was able to announce their extermination by 1937. [Brunvand, pg. 97]
There's more info, & a host of popular culture references, at this wikipedia article: Sewer Alligators.
''It's crazy, you know,'' Diana Odetalla said. ''I don't know where alligators come from. Out of nowhere, I guess.''