The Aussie film that's baffled viewers for 50 years

She had dreamed of schoolgirls in white lace, climbing across the surface of the dark monolith of Hanging Rock. They didn't belong there, on an ancient monolith, and the rock eventually let them know.

Published in 1967, Picnic at Hanging Rock sounds like science fiction, and it was - a gothic horror that sent shards of nightmarish scenes through your brain long after reading.

Film director Peter Weir, then young and relatively unknown, couldn't let the images escape either. Less than a decade later, in 1975, he wrangled these images into a film that, like the images, the mystery and the ancient monolith itself, has stood the test of time for 50 years.

It shone a new light on the Australian film industry, launched Weir's career and opened the door for generations of filmmakers and artists to work overseas.

Picnic at Hanging Rock has a plot as simple and easy to recite as, say, Jaws or The Exorcist, but the synopsis doesn't quite convey the layers of meaning. Nor does it quite sum up the atmosphere, or warn you of the chills that come on the second you recall those languid girls in ethereal white dresses, lounging in the yellowed grass at the height of an Australian summer. It's Valentine's Day, the teachers have silly parasols, the girls, in black stockings and sturdy boots, are permitted to go gloveless. Why, then, is there a pervading sense of dread in every scene?

The film endures mainly because of its atmosphere and imagery. Edwardian lace and large hats and a big country mansion. Valentine's Day swooning and later, an urgent clamouring for answers and reassurance. And then that face, of missing girl Miranda, played by Anne-Louise Lambert, is the Botticelli-like symbol - of lost innocence, of ethereal beauty vanished into the forces of nature (her iconic dress, along with other film memorabilia, is now on display at the National Film and Sound Archives). But so too is the large and looming rock - the kind of natural feature Australians have long learned to revere and respect.

It's a story that makes implicit reference to an image created much earlier by the painter Frederick McCubbin. The 1886 painting Lost depicts a young girl in the Australian bush. We only know she's lost because of the painting's title, although the way her dress and hat stand out from the all-encompassing landscape already give you that haunting sense of dread.

Weir's film, like the book, is constructed like an Impressionist painting. The girls are young and beautiful and white, in both senses; the rock is also beautiful, yet dark and ancient and forbidding. Why do Miranda, Marion, Irma and Edith rouse themselves from their summer torpor and decide to climb through its gaps and crevices? They fall asleep in a trance, and then wake up, and three of them continue climbing. The fourth, Edith, screams in terror as they drift off. Three of the girls and one of the teachers are missing, one returns with no memory of what has happened and, infected by the ever-cinematic virus of fear, panic and dread, the school steadily implodes from within.

The film, like the book, has no answers; readers and viewers have been trying to work it out for almost 60 years.

When it came out in 1975, Picnic was an instant success, both here and overseas. It shone a completely new light on the Australian narrative and the country's sense of itself, and could be absorbed by people of all ages, on varying levels. It was erotic without being explicit, scary without any gore (unlike, incidentally, the book) and suspenseful, with a masterful lack of answers.

ACM film critic Cris Kennedy says while the film was part of a "new wave" of Australian cinema in the late 1970s - think My Brilliant Career, Mad Max and Sunday Too Far Away - it has remained at the pinnacle, referenced again and again in the years since.

"I think David Lynch owes a lot to Picnic at Hanging Rock," he says.

"Almost like the characters in Twin Peaks, it looks like one thing but it's something else. There are so many layers to what appears to be a lovely, idyllic country pastoral, and yet there's darkness in the class structure."

Fifty years at the pinnacle of Australian film history is no small feat, and deserves a treat beyond a simple re-watch. The National Film and Sound Archive has now released a series of deleted scenes.

Weir's vision was whittled down to just 115 minutes, and these small scenes, including an alternative ending sequence, were consigned to the cutting-room floor. A later version of the film, released in 1988, was even shorter, in line with his original vision that was vetoed by the film's investors.

"The changes were important in retaining the overall tension of the film," Weir told the NFSA.

"The deleted scenes, when viewed in isolation, may have charm, but they contributed to a languor that was harmful to the mood I was carefully building."

Luckily for us, 50 years on, some of the 35mm film was rescued by Weir's artistic collaborator, the artist Martin Sharp, who saw the deleted scenes as the same kinds of shards and fragments of impressions and dreams that make up the finished product.

Watching them now - significantly, the scenes are silent - is like coming across a pile of old, disordered photographs. The scenes are outtakes that focus mainly on the four schoolgirls climbing up the rock, as well as symbolic imagery like waving grass, wildflowers and a white swan. The girls play and dance and, thanks to the lack of sound, seem filled with portent. Miranda, at one stage, stands in a kind of shadowy glen, looking over her shoulder. We know already, from her cryptic words early in the film, that she knows more than us; here, she seems to have one foot in another realm already.

There's also an entire end sequence that never made it to either version; the school's fearsome headmistress heads to the rock dressed in her heavy funereal blacks, and seems to be pulled in by forces similar to the girls. She sheds some garments as she heads to the top, where Sara, the girl she has tormented throughout the film to the point of suicide, is waiting in white.

It's like a glimpse into an alternate universe - the cutting room floor where visions are pared back. Not many works - of film or literature - benefit from having edited pieces reinstated, and Picnic stands alone without the extended ending. But seeing this extra vision and imagining the conversations that came before and after is tantalising.

In her biography of Lindsay released this year, Brenda Niall writes that Lindsay herself wasn't entirely sure what she had written when she finally dropped her pen.

"A novel without an ending? A mystery with no solution? An attack on an education system that stifled young women? A story of a place in which the human figures were ultimately insignificant in an ancient land that rejected European intrusion?" she writes.

As more and more people found themselves gripped by the narrative, the quest on the part of readers - and later viewers - as to the ending of the story, its origins and meaning bordered on obsessive. But Lindsay was happy to leave it to the reader.

"It encapsulated her ideas about time being all around us, not linear, leaving open the possibility that Miranda, Marion and Miss McCraw exist in a parallel time," Niall writes.

"They might even return, as if nothing had happened."

Significantly, though, Anne-Louise Lambert later recounted meeting Lindsay on the Picnic set. The older woman gripped the young actress, hugged her and said, "Miranda, it's been so long", with tears in her eyes. It made the young actress wonder whether Miranda was indeed based on a real person.

Niall posits that the book was Lindsay's attempt to reconcile the secrets and hurts of her own life - her treatment at school, her lack of children, her relationship with the landscape.

But 50 years is a long time to go without any real answers - perhaps that's the key to the film's enduring appeal. It's a perfectly executed interpretation of a finely turned novel, one that needs no tweaking whatsoever. Let audiences make of the deleted scenes - and of Lindsay's biography - what they will.

As for Weir, his career went on an upward trajectory that had little to do with Picnic, at least aesthetically. Gallipoli, Dead Poets Society, Green Card, The Truman Show, Master and Commander - there's no clear through-line in this partial list. Kennedy says this is as it should be for a jobbing film director.

"I think the link is mainly that for the first time, Australian directors are invited to Hollywood and given the opportunity to make money," he says.

"If you're an author, you get to have recurrent motifs and themes and things. If you're a screenplay writer, or a stage play writer, or a film director and you actually want to work and pay your mortgage and feed your kids, well, I think themes are things that film students find in your work afterwards."

For his part, in a recent interview with the National Film and Sound Archive, Weir said looking back was as mysterious and hard to pin down as his first experience of reading the book.

"In my memory, Picnic is like a foreign country, one I journeyed to as a young man," he said.

"A very foreign country - strange, beautiful and haunting - and only possible to visit once. The memories of that visit remain vivid and undimmed by time."

Who wouldn't want to revisit that foreign country, though? It's worth noting that there was an inevitable remake in the form of a prestige series on Netflix in 2018. The verdict?

"That was perfectly lovely, but I just don't think you can get lightning to strike twice," Kennedy says.