Was visiting this hunter-gatherer tribe in the Southern Amazon called the "Ache" (Ah-chey") back in 1999. Some of groups were first contacted by "Western Civ" as late as the mid 1970s. I went on a hunt with them in the cold jungle. A source of mortality for the Ache is jaguar predation. Jaguars have been known to eat the Ache people. Sitting around a fire at night, jaguars would pounce and bite the head of an individual and drag them back into the dark jungle and up a tree. There's pretty much nothing one can do at that point. We had to spend a night in the jungle that evening after the hunt. I was scared sh__less as the jungle was pitch black, save for the eerie fire and shadows, and there were all sorts of strange sounds. I kept waiting to get attacked and dragged back up into a tree by a jaguar. I didn't sleep, but somehow didn't feel the numerous ticks that had crawled up my pants to feed in my groin area. At dawn, we had orange slices and raw coati (similar to a raccoon) liver for breakfast. What a visit!
The above was the 2nd scariest thing I've done. The first was probably asking a girl that I was crazy about out on a date back in 8th grade.