Thanks AR.
There is some blackening visible in this image starting two or three metres in. (Bear in mind this was taken with a wide angle lens.) I wondered if this is peat leaching but there isn't any peat on the slope above. So maybe it is soot? If the slabbed over tunnel is an inlet flue, I suppose there may not be any sooting closer to the entrance.
But I'm well into the realm of speculation now. We don't seem to have reached any consensus on this one, so I think it's probably time to consult the fount of all knowledge, David Johnson. Leave it with me and if I learn anything useful I'll post here.
Meanwhile (although I suspect this is off your patch AR) if you do happen to be nearby, maybe go for a look yourself?