They are not supposed to be here. They are not native to the area, or to any area near here. Probably a few people who bought them as little lizards and changed their mind (about keeping them as pets) when the gators started to grow.

There used to be rumors of people who bought little baby alligators and then flushed them down their toilets. The gators supposedly ended up in the sewers and grew to monster sizes as they got older.