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The main points of: An Unknowable Evil in Picnic at Hanging Rock

There is no visible monster in the movie. There is no idea what any monster might be. The disappearance of the girls is never solved. (Nor is what Edith saw nor what happened to Irma.)

As the girls climbed higher in the rock they seem to be sort of in a trance, taking off some clothing, walked between some rocks and disappeared.

The movie covers what happens in the community as they search for the girls.

(One of the under-examined parts of the movie is how Irma was found days later in relatively good shape. It can be assumed that she had no access to food or water in her disappearance. Since her clothing and she are both in good shape then that would do away with one explanation of the girls being attacked by two guys and supposedly raped and then murdered. )