believability
Impossible that authorities wouldn't search for the people that go missing, and some of the hikers would have told people where they were headed before they set out. This part of reality is conveniently overlooked for the sake of a pretty good thriller.
I would have found it more believable if the area was more remote, such as the Amazon, and it was a tribe that had obviously not had contact with the outside world. The Yucatan has been well explored, and is full of tourists.
Of course, the story would have to be completely different, because a group of college kids wouldn't go on a hike by themselves in the Amazon region, so I guess there was no choice.