In the first year of the war the U-C boats discharged their cargoes of mines as soon as they could reach their respective areas of operation. The mines were usually laid close together in one field, frequently situated off some prominent headland, or at a point where trade routes converged. Then the enemy learned to respect the British minesweeping and patrol organisation, and endeavoured to lay their "sea-gulls' eggs" in waters which had been recently swept, or where sweeping forces appeared to be weak in numbers. When this failed they played their last card, scattering the mines in twos and threes over wide areas of sea. To meet this new mode of attack large numbers of shallow-draught M.L.'s were employed to scout for the mines at low water. It was about this time that the great Allied mine barriers across the entrances and exits to and from the North Sea were completed and the losses among the U and U-C boats became heavy. A rapid abatement in the submarine offensive soon became apparent, and utter failure was only a matter of time. FOOTNOTES: [8] The question of water pressures and many other problems of submarine engineering relating to under-water fighting are fully treated in _Submarine Engineering of To-day_, by the Author. [9] A few of the 7000 were British mines no longer required in the positions in which they had been laid. CHAPTER XII THE MYSTERIES OF MINESWEEPING EXPLAINED THE task which confronted the naval minesweeping organisations in the years succeeding 4th August 1914 was an appalling one. Any square yard of sea around the 1500 miles of coast-line of the British Isles might be mined at any moment of any day or night. There were, in addition, the widely scattered fields laid by surface raiders like the _Wolfe_ and the _Moewe_, which, as described in a previous chapter, extended their operations to the uttermost ends of the earth. A wonderfully efficient patrol of the danger zones had its effect in reducing the number of submarine mine-layers available to the enemy and in rendering both difficult and hazardous the successful execution of their work, but neither a predominant and subsequently victorious fleet, nor an equally skilful and alert patrol, could guarantee the immunity of any considerable area of sea from mines.