I passed out is West Kennet Longbarrow once.
I was stood in the main chamber facing the entrance staring straigt up at the ceiling. Blue sparkly bits appeared in my vision, and out of interest I kept staring. Next thing I know I'm bouncing of the walls, which feel all spongey and i hear the phasey-echoing laugh of my friend. Thats when I regained my posture, sat down and pondered.

Was I in contact with some unseen entity? Or had I visited Avebury at the end of a two week acid binge?

Definitely the latter, i like to think it was also the former.