I had heard a rumor that the missing ending actually implicated a UFO, possibly aliens kidnapping the girls, but the summary clearly disproves that. Wikipedia notes that the most common explanation (by fans and critics, I guess) is that the girls stumbled into "a time warp." I can't speak for the book, obviously, but I got the feeling (with the movie) that there was something having to do with female sexuality and civilization encroaching where it has no business--and no clue. The civilization bit is self-evident, given the way people are having picnics and parties out in the frickin' Australian outback; the female sexuality part is more from the beginning of the movie, with it being Valentine's Day and the girls being all languorous and full of poetry and Sara obviously having a huge crush on Miranda. But also--the headmistress says something later about Miss McCraw that seriously makes me think they were in some kind of informal relationship. Nothing huge; she just gets angry at dinner with the French teacher, seems to be a little drunk, mentions Miss McCraw by her first name, and says something along the line of, "How could she be so stupid?" Kind of in the sense of, "How could she be such a girl about it and get herself killed?" (Seriously, if you remember the actual line, help me out here.) I mean, clearly it could just be that the headmistress admires her, but for some reason, I seriously got the feeling that they were in some kind of relationship, which the Sara/Miranda thing kind of supports as being possible--as in, the movie's not afraid to go there.
Anyway, I always kind of thought (and I seem to remember defending in a film class paper in college) that the mountain swallowed them up because it was a (super)natural hot spot, for lack of a better word--a hot spot aggravated by 1) the cavalier way the English schoolgirls (and the family with the two boys, the son and the servant) intrude with their picnic and 2) the intense female adolescent sexuality roiling around. It's the idea that there are just places in nature where it's better not to trespass, where you have no idea what you're getting into, which is exactly what happens with the daytrippers--here's this fancy girls' school out in the middle of frickin' nowhere, in the bush, which is a supreme act of nose-thumbing defiance against the wilderness, if you think about it. And part of the reason I like my explanation is that I cannot imagine the rock actually doing anything. Maybe it lures the girls up there, and then into the crevice, but I can't imagine what it "does" with them afterwards. Or why it doesn't take the fourth girl. I don't really know what happened to Miss McCraw, either--why Edith saw her without her dress, or what happened to her afterwards. And that's why I like it, because I don't know, because that's the nature of the rock. It's something you can't know, something beyond reason. If you had been on the rock watching when the girls disappeared, you would have been temporarily blinded and unable to see what happened. In fact, Edith can't explain or recall what happened after the girls walked into the rock(s), like either time skipped or she was plunged into a hysteria of unreason or something.
So on that same web page, I read someone's theory as to what actually happened to the girls. And I have to tell you, it pisses me off because the guy supports it so well--pissed off in that sense of "NO NO I AM NOT WRONG SHUT UP." You know, when you know you're totally beaten at your own game. His theory is that it was a rock slide, a completely normal, unsupernatural rock slide, and that Lindsay's overarching simile is rock is to girls as girls are to ants, particularly in the sense of the girls stepping on ants and crushing them without knowing it or caring. And pretty much all the evidence he presents points to a rock slide--a red cloud of dust, a rumbling sound, footprints obliterated by rubble, watches stopping as if before an earthquake (the latter idea supported by something Lindsay wrote previously to that effect)--although I'm still not sure how Irma could have taken her corset off in the process of digging out, and I don't know that this explains Miss McCraw being seen running up to the rock already in her drawers, but then I don't know if that second part was only in the movie, as (again), I haven't read the book yet. But just the theory that Lindsay's similes and description of the rock are intended as clues to the rock slide--well, it's depressingly well-supported.
The theory finishes with this:
If you believe that Joan Lindsay wrote “Picnic at Hanging Rock” knowing what happened to the missing girls and Miss McCraw, then the solution documented here, or a close variation of it, can be the only plausible solution. This solution is consistent and believable, and can be justified by numerous quotes from the novel.And then you get the summary of how Lindsay actually solved the mystery herself (which the theorist implies he had not yet read at that time), which is basically that Edith runs off and the girls go to sleep and they wake up and there's Miss McCraw but they don't recognize her and they all go kind of giddy and weird and take off their corsets and throw them over the cliff and Miss McCraw is all like, "Whoa! Look at that shit!" and the corsets are floating in the air suspended in space and time and then there's a hole (in space) (and time) that, like, totally has all the answers to the mysteries of life and a small brown snake slithers out and disappears down a crack in the rock and Miss McCraw is like, "We should totally follow that snake," and the girls are like, "We should what in the where now?," and Miss McCraw turns into a snake and follows the other one, and Miranda and Marion are like, "Dude," and they turn into snakes and slither after her, and then a boulder falls over and covers the crack and poor Irma is left there all like "WHAT."
The mystery does not have to be explained using inconsistent, supernatural, or far-fetched reasoning! Attempts at doing this degrade the reputation of both Joan Lindsay and of the novel itself, which has charmed readers since 1967.
Oh, the hilarity.