For reference on this thread see also the links to John Skinner's words posted at - http://www.themodernantiquarian.com/forum/?thread=45741&message=572706

"...he came to a mouth of a passage covered with a square stone similar to that at (nearby) Plasne-wydd, anxious to reap the fruits of his discovery he procured a light and crept forward on his hands and knees along the dreary vault, when lo! In a chamber at the further end a figure in white seemed to forbid his approach. The poor man had scarcely power sufficient to crawl backwards out of this den of spirits..."

Very similar.