I was adding a Stonehenge gallery to the THUMBNAIL TRAVELER Facebook page and ran across this shot. It won't mean much to anyone outside of fans of Doctor Who, but this appears to be the doorway to the Pandorica Chamber.
I asked the guide escorting us through the Henge about the doorway, and he laughed, pointing conspiratorially towards this area.
Naturally I had to take the picture. My theory is it's the panel on the left at the base of the stone.