Stone Age people living in cave? In the 20th century? You can not believe it? Then you are right, it is not true. But anybody believed it, even National Geographic brought a title story about the Tasaday tribe.
The Tasaday had been living in their caves in the rain forest for over one thousand years without contact to any outsider. Their isolation ended when a hunter from a neighboring tribe stumbled upon them while laying his traps. Or so the story goes. The whole thing is mysterious. But there are several very detailed web pages, so help yourself.