***********This might have some spoilers, so be careful.************ (Funny, spoilers for a story.. that seems different.
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I'm pretty sure that all of the "mystery" part is a work of fiction. And even if it isn't... I'd rather believe otherwise.
It is told very well, though. I especially like how everything from the name they give the cave to all the explanation of caving he gives on the fourth page ties in to what happens to them in the end. And I love how much suspense builds up just by the very calm and thorough writing and subtle hints to what will happen in the end.
Though I was glued to my chair through the whole thing, I think the things that gave it away are, like Danchastu said, the photos of the cave magically don't appear and the video camera is left behind (so absolutely no evidence). Also the guy just feels compelled to go to the lookout to see the city lights and mysteriously meets Joe there. And then conveniently they never gave the name of the cave to anyone so even if someone wanted to find them they couldn't. (When they expect to be in enough danger to carry a gun then you would think they would definitely tell at least one person so they could be found.)
Even despite that, with all the immense amount of detail given about the cave some other caver would know what cave it was. And this sort of thing would be all over the news; this is the kind of story the presses couldn't wait to break.
But since those pictures seem very real and the hard work they went through and documented seems real, I'm guessing they found this hole, wanted to explore it like they said, opened it up, went through, found nothing interesting, and then made up a cool little story.
My theory, anyway.
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