The Devil's Den is a bit of a curiosity. It is hard to find and involves hiking through fields with an ordnance survey map, as it is on private land and not signposted. It is a dolmen burial chamber, or at least looks like one now, with two standing stones and a capstone on top, but closer inspection reveals some rather modern cement holding the upper stone in place. Apparently this is all that is left of what was once a 230 foot long barrow that has since been ploughed over.
I found this tiny nature reserve on the Wiltshire Wildlife Trust website when I was looking for places to visit near Marlborough and Savernake Forest. I am always on the lookout for places that are not well known, which is in keeping with this sites ethos to explore "hidden Wiltshire". At less than one hectare in size it is extremely small and there isn't any signage at the site to indicate there is anything of note. It is actually a small section of dismantled railway, whic