Well, you could go with a good ol' crazy native cult of your own invention (though, of course, its harder to do this now without it being immensely politically incorrect depending on the area of the world you are in). That, or perhaps some bad guys (crazy mercenaries, Neo-Nazis, members of an ancient secret society) have gotten stranded in the same area your heroes are in, and the two groups inevitably meet and clash. Or you could even get really crazy and have the environment itself be cursed somehow, and so the threat comes from supernatural threats spawned by the curse, like mummy warriors or demons assigned to guard the prize.
Just some ideas.