Early in the war the British were astonished to find enemy mine-fields in their own waters, far from any German ports. They could not have been planted by surface mine-layers, unless these had managed to creep up disguised as peaceful trawlers. This seemed hardly likely, because these fields appeared in places that were well guarded. Then it was discovered that German U-boats were doing this work. Special mine-laying U-boats had been built and one of them was captured with its cargo of "devil's eggs." A sectional view of the mine-laying U-boat is shown opposite page 272. In the after part of the boat were mine-chutes in each of which three mines were stored. A mine-laying submarine would carry about a score of mines. These could be released one at a time. The mine with its anchor would drop to the bottom. As soon as it struck, anchor-arms would be tripped and spread out to catch in the sand or mud, while the mine cable would be released and the mine would rise as far as the cable would allow it. The U-boat commander would have to know the depth of water in which the mines were to be laid and adjust the cables to this depth in advance. This could not be done while the U-boat was submerged. With the mines all set for the depth at a certain spot, the U-boat commander had to find that very spot to lay his "eggs," otherwise they would either lie too deep to do any harm to shipping, or else they would reach up to the surface, where they might be discovered by the Allied patrols. As he had to do his navigating blindly, by dead-reckoning, it was very difficult for him to locate his mine-fields properly. But the Germans did not have a monopoly on submarine mine-laying. The British also laid mines by submarine within German harbors and channels, right under the guns of Heligoland, and many a U-boat was destroyed by such mines within its home waters. [Illustration: (C) Press Illustrating Service A Dutch Mine-sweeper engaged in clearing the North Sea of German Mines] PARAVANES