A subterranean passage was said to exist between the abbey [of Hailes] and Coscomb, and also to Beckbury, the camp above Hailes Wood; but no one in the present day can point out the whereabouts.
From the Transactions of the Bristol and Gloucestershire Archaeological Society for 1884-5.
Another story (possibly apocryphal) is that Thomas Cromwell sat up on the ramparts of Beckbury to watch the destruction of Hailes Abbey. Perhaps he came up the passage?
Oh, good one. Possibly he did :) I always think mysterious underground passageways are one of the best sorts of landscapey folklore.