They are big. They are vicious. Some say they are albinos, because of a lack of sunlight.They are the alligators that supposedly infest New York City’s sewer system, slithering through the bowels just under the street level, feeding on rats and rubbish and terrorizing sewer workers armed with guns for self-defense.These gators may be the city’s most entrenched urban myth, one that has permeated pop culture and has become a recurring theme in books, television shows and movies.With every new sighting, the legend gets another boost.