nly a few urban legends have a "distinct local flavor or occasional detail," says Jan Harold Brunvand, professor emeritus, folklore specialist and author of seven books about urban legends. "Most urban legends travel freely between different cities and even different countries," he says.

The legend of alligators in the sewers of New York is one of the rare urban legends that stick almost exclusively to one city, he says.

According to Brunvand, many of the stories convey messages or warnings directly (check the back seat of your car before you get in) or more often indirectly (you can’t trust big business or gangs are everywhere out there).

Even though most of the stories are — as the title of one of Brunvand’s books says — "too good to be true," they keep being told because they are appealing, funny, bizarre, suspenseful and sometimes shocking, he says. "The truth never stands in the way of a good story."

Most of the stories could even have a news value, according to Brunvand. "Strangely, few seem to question why this news is just floating around word-of-mouth or on the Internet instead of being reported by the professional media," he says.

"It always happened to a friend of a friend," says Nancy Groce, folklorist and curator at the Smithsonian in Washington D.C.. "But if you ask that friend, it didn’t happen to him either — but to a cousin. That is how you know it’s an urban legend."

Of other local folktales tied to New York, Groce gives the examples of New Yorkers who try to sell the Brooklyn Bridge to newcomers in the city, claiming the land used to belong to their great great grandparents. And the tale of Steve Brody, who supposedly was the first person to jump off the Brooklyn Bridge and survive. "He lived," says Groce, "but whether he jumped the bridge…"

It doesn’t really matter whether it’s true, she says. "From a folklore standpoint, what is interesting is why do people continue to tell them. It’s because they give a voice to what concerns us," she explains.

The stories tell us something about the development in the society, Groce says. They are, for example, no longer about the Black Death and the plague — but about fear of medical development.

Kidney theft, for example — in short: A young man visiting the city is picked up by an attractive woman, wakes up the day after doped up and he realizes his kidney has been stolen. This legend has traveled around the world, some say tell it as happening in New York City, Groce says, but people change the location to give the story validation.

According to Brunvand, the glory days of the urban legends were roughly the 1960s through the 1980s — though the stories existed both before and after this period. But this era was the high point of the legends being told by the "face-to-face processes of word-of-mouth transmission," he explains.

"Urban legends have mostly migrated from folklore into popular culture where they have become stereotyped, standardized, exploited, commodified and repackaged in a number of ways," Brunvand says.

Today, many of these stories are circulated via the Internet.