Picnic at Hanging Rock (1975)

Three students and a school teacher disappear on an excursion to Hanging Rock, in Victoria, on Valentine's Day, 1900. Widely (and incorrectly) regarded as being based on a true story, the movie follows those that disappeared, and those that stayed behind, but it delights in the asking of questions, not the answering of them.

http://www.imdb.com/title/tt0073540/