I have a puzzle about the Landings. A huge invasion effort was assembled on the South coast of England over a long time. Huge numbers of troops, all in uniform, milling around; new barracks to house them; armoured vehicles, tanks, landing craft, Mulberry stuff — hardly inconspicuous, PLUTO (oil pipeline). I believe that the Germans knew that an invasion was planned, but how come (as I understand it) the first they knew of it was when the invasion was half way across the Channel?!
Arguments that I have heard explaining this include no photo reconnaissance because the Luftwaffe did not have air superiority and no satellites; German spies were caught soon after landing and therefore were unable to warn; communications were cruder in those days, only gossip but no Internet helping leaks; The UK had a a friendly population which was unlikely to blab; skilful camouflage distraction hiding the activity; the Germans had the whole of the South Coast to watch.
Still, I am still surprised that they had no inkling of such a vast preparatory operation lasting for a long time. Any ideas?