I thought they were an urban legend.

Actually sometimes things get blown way out of proportion and even factual events will sometimes become an Urban Legend.

Modern sighting

In February 1935 a large alligator was reported by the New York Times as being discovered in a New York City sewer. According to the story, several teenage boys were disposing of snow into a manhole when they spotted an alligator, allegedly 7 feet (~2.1 m) long, that had gotten stuck in icy water. The male youths then dragged the trapped reptile to the surface. After the alligator snapped at one of them, the teenagers beat it to death with their snow shovels. The report suggested that the alligator had escaped from a ship traveling from the Everglades and had then swam into the Harlem River and then 150 yards (~137 m) up a storm conduit to where it was found.[1][1]

Sewer reports

That same year reports were given to the city's Superintendent of Sewers, Teddy May, that swarms of alligators were thriving beneath the city. May, convinced that the men filing the reports were drinking on the job, took the suggested sightings lightly. It was not until he found that there was no real drinking of alcoholic beverages taking place in the sewer that he followed up the claims. To his shock, he witnessed a large number of alligators, most only about 2 feet (61 cm), to be living within pipes that emptied into the trunk lines below major streets.[2]

Sewer clean-up

All the reptiles were apparently exterminated within a few months, killed mostly using rat poison, flushing them out to sea through trunk lines or even shooting them.