According to German testimony, mines were responsible for the failure of the U-boat. However, it was not merely the scattered mine-fields sown in German waters that brought the U-boat to terms, but an enormous mine-field stretching across the North Sea from the Orkney Islands to the coast of Norway. Early in the war, U-boats had been prevented from entering the English Channel by nets and mines stretched across the Straits of Dover. As the submarine menace grew, it was urged that a similar net be stretched across the North Sea to pen the U-boats in. But it seemed like a stupendous task. The distance across at the narrowest point is nearly two hundred and fifty miles. It would not have been necessary to have the net come to the surface. It could just as well have been anchored so that its upper edge would be covered with thirty feet of water. Surface vessels could then have sailed over it without trouble and submarines could not have passed over it without showing themselves to patrolling destroyers. It would not have been necessary to carry the net to the bottom of the sea. A belt of netting a hundred and fifty feet wide would have made an effective bar to the passage of U-boats. As U-boats might cut their way through the net, it was proposed to mount bombs or mines on them which would explode on contact and destroy any submarine that tried to pass. However, laying a net two hundred feet long even when it is laid in sections, is no small job, but when the net is loaded with contact mines, the difficulty of the work may be well imagined. And yet had it been thought that the net would be a success it would have been laid anyhow, but it was argued that seaweed would clog the meshes of the net and ocean currents would tear gaps in it. Even if it had not been torn away, the tidal currents would have swept it down and borne it under so far that U-boats could have passed over it in safety without coming to the surface. A WALL OF MINES When America entered the war, we were very insistent that something must be done to block the North Sea, and we proposed that a barrage of anchored mines be stretched across the sea and that these mines be set at different levels so as to make a "wall" that submarines could not dive under. This would do away with all the drawbacks of a net. Ocean currents and masses of seaweed could not affect individual mines as they would a net. Furthermore, an American inventor had devised a new type of mine which was peculiarly adapted to the proposed mine barrage. It had a firing-mechanism that was very sensitive and the mine had twice the reach of any other.