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You might start by creating a superstition, and back it up by a setup consisting of motion detectors and infrasound.

The motion detectors would help in activating the infrasound only when people are approaching, thus allowing the local fauna to exist undisturbed. You could let it work on animals, too, if you like, since seeing that even animals avoid the region might even strengthen your superstition. On the downside, it could be a cause of curiosity.

According to Wikipedia (http://en.wikipedia.org/wiki/Infrasound), Infrasound will inflict all kinds of bad feelings, and might even cause ghost sightings. This should make sure anyone doubting the superstition and trying to see for themselves should be scared off.

If necessary, a second line of defense might be an actual weapons array, making sure anyone making it through the infrasound barrier does not return to tell the story.

Combined with the scarcity of ressources and the resulting scarcity of explorers (they would be busy surviving most of the time) this should provide the secrecy you are looking for.