Reader's Digest documentary exploring the mysterious natural and man-made structures that lurk beneath the surface of Britain. 'Hidden History' looks at the role underground structures have played in the history of Britain from the ancient past to the present. 'Wartime Secrets' examines the various underground structures that were key to the Second World War, including bomb shelters and military command centres. 'Modern Mysteries' explores comparatively modern underground spaces, especially the bunkers created during the Cold War for use in the case of a nuclear emergency.