Once the torpedo-boat sharply ported her helm. For the moment it seemed as if the submarine had been spotted, but since no shell was fired from the Turkish craft the Lieutenant-Commander surmised that the change of course had been rendered necessary by the intricacies of the secret passage through the mine-field. Allowing sufficient distance the submarine followed suit, until the torpedo-boat swung round on her former course. She had cleared the danger-zone, and her imitator had done likewise. Slowly but surely the Turkish craft was out-distancing the invisible submarine, whose utmost limit when submerged was not equal to that of the torpedo-boat. Half an hour later all traces of her had disappeared. Even the churned wake had blended utterly with the surrounding waves. The submarine was now about to enter the Sea of Marmora. The search-lights of Gallipoli were broad on the port beam. On the port bow a row of flickering lights marked the camp-fires in the Bulair lines. A triple row of anchored mines had to be avoided before the British craft was clear of the upper reaches of the Dardanelles. Down she went till the gauge indicated a depth of fifty-five feet, then regaining an even keel she forged slowly ahead, deliberately feeling her way through the black water. A peculiar rasping, muffled sound attracted Dick's attention. The Lieutenant-Commander heard it too. The two men exchanged glances. "One line passed," announced Huxtable when the noise ceased. "That was the mooring wire of one of the mines scraping along our protective girders." Ten seconds later came a similar sound, this time overhead. The submarine was passing under a horizontal bridle connecting two mines. The arched girder saved her, for had the obstruction caught in the for'ard end of the conning-tower or the housed periscope, the "way" of the vessel would have swung the two mines together with annihilating effects. As it was, the Lieutenant-Commander merely depressed the boat's bows, and without any trouble the submerged craft glided underneath the cable of death. "Two!" ejaculated the skipper laconically. Dick's spirits rose rapidly. After all, he reasoned, diving under mines was an exciting form of sport, with very little danger. The operation seemed far less hazardous than running the gauntlet of The Narrows in the _Calder_. Suddenly came the dull roar of an explosion. The submarine heeled dangerously, and quivered till it seemed that her plating was on the point of buckling. Instinctively Crosthwaite grasped the hand-rail of the steel ladder leading to the conning-tower hatch-way. In any case it was a futile action, since there was no possible escape that way. He momentarily expected to hear the rush of water, driven under tremendous pressure into the shattered hull. The submarine had come in contact with a mine. CHAPTER XV Disabled