The Adriatic Sea was an ideal place for the use of the hydrophone. The water there is so deep that submarines dared not rest on the bottom, but had to keep moving, and so they could easily be followed. Across the sea, at the heel of the boot of Italy, a barrage of boats was established. U-boats would come down to this barrage at night and, when within two or three miles of the boats, dive and pass under them. But when hydrophones were used that game proved very hazardous. Our listeners would hear them coming when they were miles away. Then they would hear them shift from oil-to electric-drive and plunge under the surface. Darkness was no protection to the U-boats. The sound-detector worked just as well at night as in the daytime and a group of three boats would drop a pattern of bombs that would send the U-boat to the bottom. On one occasion after an attack it was evident that the submarine had been seriously injured. Its motors were operating, but something must have gone wrong with its steering-gear, or its ballast-chambers may have been flooded, because it kept going down and soon the listeners heard a crunching noise as it was crushed by the tremendous pressure of the water. And so U-boat warfare grew more and more terrible for _Herr Kommandant_. The depths of the sea were growing even more dangerous than the surface. On every hand he was losing out. He had tried to master the sea without mastering the surface of the sea. But he can never really master who dares not fight out in the open. For a time, the German did prevail, but his adversaries were quick to see his deficiencies and, by playing upon these, to rob the terror of the sea of his powers. And as _Herr Kommandant_ looks back at the time when he stepped into the lime-light as the most brutal destroyer the world has ever seen, he cannot take much satisfaction in reflecting that the sum total of his efforts was to spread hatred of Germany throughout the world, to summon into the conflict a great nation whose armies turned the tide of victory against his soldiers, and finally to subject his navy, second only to that of Great Britain, to the most humiliating surrender the world has ever seen. CHAPTER XIV "DEVIL'S EGGS" In modern warfare a duel between fixed forts and floating forts is almost certain to end in a draw. Because the former are fixed they make good targets, while the war-ship, being able to move about, can dodge the shell that are fired against it. On the other hand, a fort on land can stand a great deal of pounding and each of its guns must be put out of action individually, before it is subdued, while the fort that is afloat runs the risk of being sunk with a few well-directed shots.