Two decades ago, 118-year-old Morningside Park was among the most dangerous parks in the United States. Thousands of empty crack vials littered the forested pathways. Decrepit buildings lined Morningside Avenue. Columbia University warned students not to enter the park after dark. (In 1968, Columbia had attempted unsuccessfully to turn the park into an athletic complex.) Things, as they say, have changed. Perhaps the most obvious signal of the park’s rebirth is the little pond at its center. The pond is a mecca for the ducks, turtles, and Canada geese that jostle for control of the single sunning rock that juts from the water’s calm surface, and for the young families, dog walkers, and aimless wanderers who take in the bucolic scene. Its signature element is the waterfall, big and impressive, that flows off rugged cliffs that designers Frederick Law Olmsted and Calvert Vaux wisely left in place.