The plot: An anthropologist travels to the Amazon to determine the fate of a missing American documentary crew. He finds the tapes left behind by the murdered crew and reveals how and why they were murdered.
What goes bump in the night: Angry Amazonian villagers. Though the behavior of the American crew is just as shocking and reprehensible. So let's say the monster here is... us!
How the footage was found: It was being held by the Amazonian tribe, traded to the anthropologist through a cannibalistic ceremony.
Fun fact: Ten days after the film's premiere, director Ruggero Deodato was arrested on obscenity charges, which were later amended to include a murder charge. It was only after Deodato assembled the film's cast for a TV appearance and explained how the film's graphic effects were achieved that the murder charges were dropped. Deodato was convicted of obscenity, however, and the film was banned for years in many countries.