Unseen Evil (2001)

Professor Peter Jensen hires a Native American guide to lead him and a few associates to the sight of some rumored hieroglyphics.  Jensen is cantankerous and rude to the guide.  He wants to keep the trip a secret because he plans to steal Native American artifacts from the cave he's seeking.

On the drive, their guide stops at the ranger station to let the ranger of their plans. This is unfortunate because the muscle that Jensen's hired decides to make sure the ranger won't talk and conks him on the head.

While trekking through the woods, the thugs guns and Jensens plans are inadvertently revealed.  The guide and female member of Jensens party (who wasn't on the plan) are restrained and brought with them so Jensens nefarious activity can't be reported.

The problem Jensen didn't count on is there's an invisible monster in the cave who isn't very happy about being disturbed or the theft.  Also the artifacts are the cheapest looking relics ever. They appear to be plastic.   Either we're not supposed to notice, or Jensen is a terrible archaeologist.

This low budget film has Tim Thomerson in a brief role as the ranger and Richard Hatch, aka Apollo from the 1970s Battlestar Gallactica series, as Professor Jensen.  The invisible monster is occasionally visible and is a ridiculous looking, cartoonish CGI creature that is incredibly unrealistic.  And as if the invisible monster isn't silly enough, they even toss a mention of UFOs into the mix.