Research suggests that much of what these substances do is induce a waking dream state. As I noted in a previous post, our dreams are the most common examples a brush with what C.G. Jung called 'the collective unconscious,' where the archetypal, archaic images hold domain. A brush with the collective unconscious always carries with it an experience of something of the numinous, which more often than not brings in its wake elements of both terror and ecstasy.
Here's a recent article that may resonate with you.