In this paper, we present a novel hypothesis as to what led humans in the Upper Paleolithic to penetrate and decorate deep, dark caves. Many of the depictions in these caves are located in halls or narrow passages deep in the interior, navigable only with arti!cial light. We simulated the effect of torches on oxygen concentrations in structures similar to Paleolithic decorated caves and showed that the oxygen quickly decreased to levels known to induce a state of hypoxia. Hypoxia increases the release of dopamine in the brain, resulting in hallucinations and out-of-body experiences. We discuss the signi!cance of caves in indigenous world views and contend that entering these deep, dark environments was a conscious choice, motivated by an understanding of the transformative nature of an underground, oxygen depleted space. The cave environment was conceived as both a liminal space and an ontological arena, allowing early humans to maintain their connectedness with the cosmos. It was not the decoration that rendered the caves significant; rather, the significance of the chosen caves was the reason for their decoration.