“As do other shamanistic peoples throughout the world, the San [or “bushmen”] believe in a realm above and another below the surface of the world on which they live… Concepts of a tiered universe are, of course, not restricted to shamanistic religions. Heaven above, Hell below, and the level of anxious humanity in-between appear in one form or another across the globe. Why should this be so?

…The answer to this question is, I argue, to be found in a set of widely reported mental experiences. These reports come not only from laboratory experiments but also from an extremely broad range of shamanistic (and other) societies.

…In Chapter 4, I described the sensation of passing through a vortex, or tunnel, as subjects move along the intensified spectrum and into Stage 3 of deeply altered consciousness. Tunnel experiences also occur in dreams and near-death experiences. Often there is a bright light at the end of the tunnel.

…Here, I suggest, is the reason why so many peoples around the world believe in passing underground to a subterranean realm. The notion has its origin in altered states of consciousness and then becomes part of socially transmitted culture so that even those who have never experienced the far end of the intensified trajectory accept the beliefs.

…These experiences are the result of the way in which the human brain and nervous system are neurologically constructed and the ways in which they operate electro-chemically in altered states of consciousness. To this extent, the experiences are universal. The precise ways in which they are rationalized are culturally situated and hence differ in some ways from society to society: some people speak of entering caves, others of following the roots of a tree, and still others of going down animal burrows, rather like Alice.

…Another type of sensation also derives from the structure and functioning of the human nervous system. Subjects experience weightlessness and a sensation of rising up that is often associated with attenuation. They feel that they are looking down on their surroundings and that their limbs and bodies are exceedingly long. Throughout the world, these experiences are, understandably enough, rationalized as floating and flying.The most obvious explanation of these sensations is that the subject is flying through the air. Shamanistic flight is, of course, as widely reported as underground journeys.”

— David Lewis-Williams, The Mind in the Cave: Consciousness and the Origins of Art