New York City, like all metropolises (metropoli?) of it's size, has it's fair share of drug busts. According to the urban legend, so many people have flushed their pot during these drug busts that a pretty good crop of marijuana is growing in New York's sewers. It's fertilized by... well, let's just say, all the solid human waste... and since there isn't any sunlight down there, it has mutated into a special albino breed of pot. This "New York White" is incredibly potent, and incredibly expensive because it's so hard to harvest. And why is it so hard to harvest?
Because of the alligators in the sewers, of course! In the 1980's, baby gators were trendy pets and all the yuppies had to have one. But alligators, even babies, are pretty contrary and do not make warm cozy pets. They're cute but mean, and when people realized that 1)their pet did not love them, and had no qualms about biting the bejeezus out of their owners on a regular basis; and 2)would be vicious and dangerous as adults; they decided to get rid of them. But you can't exactly release an alligator into the wild, and they didn't know how to get rid of their pets, so they finally just flushed them down the toilets. (How could they!?! The heartless bastards!) A lot of the alligators survived, and grew to enormous size feeding on sewer rats (and smoking New York White?). After a few generations, they became blind and albino and are there to this very day, biting sewer workers and making impossible for stoners to get any of the fabled weed.
(Obviously, none of this could ever happen: I have it on very good authority from an avid pothead of my acquaintance that the growing conditions in sewers would result in "doo-doo weed", and though supposedly a couple of alligators were found in the sewers in the 1940's or thereabouts, there aren't any anymore, and nobody knows how the actual alligators ended up there.)