“Extra/ultra/alternative/multi-dimensional phenomena/beings, indigenous to existent natural systems of which we are a part but that we can only perceive to a limited extent”

Open to any interpretations, but I think this is the most likely. Anecdotally from being around the psychedelic scene there is a very high % of people who seem to encounter aliens in an altered state of perception. This doesn’t automatically make them “real” but it does fit the theory. The ET-little green men from other planets way of looking of things (very problematic) also gets explained away this way. Valee’s work showing the correlations of spiritual experience through the ages and UFO sightings makes a lot of sense.

But I do think it’s likely whatever beings that exist in a imperceivable state by humans, are capable of transitioning from time to time into normal 3D. See the Ariel phenomenon (by far the most convincing case I’ve seen) and reports of those working in nuclear facilities. There’s something going on with these interactions in unaltered states of consciousness that make me think it’s a both/and instead of simply existing in one dimensions. The “aliens” are probably able to transition between two or more dimensions as they see fit.