On a cold February night in 1944, two British frogmen crawled on to a Normandy beach from the freezing sea to take samples of sand for scientific analysis from under the noses of German sentries. It was one of the most audacious of all the incredible operations that went into the planning of the Allied invasion of Europe. Throughout Britain during the 12 months before June 1944, men had been searching for the weak points in the vast German defences - all to ensure that D-Day, when it came, would be successful. The late Sir Huw Wheldon, then a major in the 6th Airborne Division, landed with his unit on 6 June to help defend the left flank of the invasion force against counter-attack. In this programme he tells the story of the Allied plans and preparations which helped ensure the success of Operation Overlord.