"They are the monsters under the bed when you are little, the shape just caught out of the corner of your eye when you thought you were alone, the shadow of something in a dark corner that surprises you and then isn't there. They stop you dead with a knot of unexpected terror in the pit of your stomach. We have all seen fleeting glimpses of them. Never long enough to see them as I saw them, but it was them. I recognized it the instant I saw it. We've...