A biogenetic structural explanation is offered for the cross‐culturally common mystical experience called portalling, the experience of moving from one reality to another via a tunnel, door, aperture, hole, or the like. The experience may be evoked in shamanistic and meditative practice by concentration upon a portalling device (mirror, mandala, labyrinth, skrying bowl, pool of water, etc.). Realization of the portalling experience is shown to be fundamental to the phenomenology underlying multiple reality cosmologies in traditional cultures and is explained in (...) terms of radical re‐entrainment of the neurological systems mediating experience in the brain. Phenomenological experiments with mirror portalling devices from both the Tibetan and the Tsimshian religious traditions are reported. (shrink)