In his first novel, "V.," Thomas Pynchon envisions an "alligator patrol" slogging through the sewers beneath New York City, hunting down beasts in that subterranean dusk. This was an old idea before Pynchon got hold of it, and it's surprising someone hasn't turned it into a full-fledged exploitation movie until now. Alligator is just that: In fact, not content with a regular reptile, it gives us a 'gater or monstrous size, nurtured on a chemical soup like the "food of the gods" in H. G. Well's story. The screenplay is by John Sayles, who made "Return of the Secaucus Seven" and wrote "The Howling." He has a sense of humor and social awareness, but in his new epic -- even more forgetabble than the others -- these qualities are swamped by the stale story and chomp-chomp violence. It's just another horror movie, and Sayles will have to come up with something more substantial if he wants to fulfill his early promise.