At a glance the view could be of any beach or cliff or headland, on the edge of any northern sea: cold and hard, with big horizons, rocks oiled with salt and moisture. Let that glance linger, though, and you start to see them. A slot in a cliff that’s a little too straight to be natural. Boulders on the beach that, with scrutiny, aren’t boulders at all. That line of rocks exposed at low tide spaced with tell-tale regularity, straight edges cut by human hand. 

During the European conflict of World War Two, the waters around the British coastline were both its savior, and its warily-watched curse. Stealth attacks by sea could come from any direction, and