The Last Wave (1977)
Director: Peter Weir
Peter Weir made this a few years after the incredibly mysterious Picnic at Hanging Rock. While the mystery at the centre of that film is never answered, the events are understandable. In The Last Wave the unsolved mystery is far more inpenetrable; I still am not sure exactly what happened (and I watched this a few weeks ago).
While it is not as successful as Picnic, there is a lot to appreciate in The Last Wave. The bizarre weather sequences are wonderfully executed. A scene where Richard Chamberlain's David looks out his car to see the outside world drowned in water is striking. In fact, what I remember about the film is how it looked. The opening scene is one of the best I have seen, setting the unsettling mood from the get-go. The acting is pretty good, David Gulpilil the standout as Chris Lee, who tries to inform David about this world.
I had not really heard of this film before seeing it. It is included in the 1001 Movies, but seems to have been overshadowed by Weir's Picnic. The Last Wave is definitely worth your time, though if you can't cope with unanswered mystery you may find it frustrating.