A replica of a painting of bison from the Cave of Altamira in Cantabria, Spain
© Getty
Stone Age people may have deliberately ventured into oxygen-depleted caves to paint while having out-of-body experiences. In the 19th century, researchers discovered a series of decorated caves that date back between 14,000 and 40,000 years to the Upper Paleolithic era, or Late Stone Age, across Western Europe. Found mainly in Spain and France, they were filled with paintings, many of them in areas that could be accessed only through narrow passageways. The depictions were painted in black and red and primarily showed animals, with some hand stencils, handprints and geometric abstract signs.