"No depth bomb ever made that upheaval," Dave gasped, as soon as he could speak, and Mr. Ormsby, much shaken, had picked himself up. "The bombs are ugly affairs, but that felt like the explosion of about ten of them." "Did you notice, sir, that the explosion lasted more than twice as long as we've ever known one to last before?" the watch officer asked. "Yes." "Then what happened, sir?" "Either our explosion touched off a torpedo, which does not seem likely, or else--" For an instant what he was about to say sounded so absurd that Darrin hesitated. "Well, sir--?" queried Ormsby. "Or else that was a mine-layer, with a full cargo of mines aboard, all ready for business, and--But you'll think I've gone daffy." "No, I won't, sir; not after the way this ship rose out of the water," the watch officer declared. "You mean, sir, that our bomb went off right over that craft's cargo of mines, and that the shock must have set off the mines." "That's certainly the way it looks to me," Darrin nodded. "I believe it, sir." Just a few moments later the patrol boat came within hail. Through his megaphone Darrin stated what he believed had taken place. "It's the only thing to account, sir, for such a tremendous explosion," replied the commander of the patrol. "I've been on hand to see a lot of depth bombs go off, and I never saw an upheaval like the one you produced, sir." "Have soundings taken, Mr. Ormsby," Darrin directed. The depth of the water was quickly reported. Dave glanced at the sky. "The light will be strong enough for another hour," he decided. "Have our two divers prepare to go down at once." A launch, cleared away with the divers on board, was anchored in the middle of the oil spot. Two divers went over the side. Presently they signalled for extra cables. When these were let down they attached pieces of metal and gave the signal to haul away. By the time that the hour was up Darrin had abundant evidence to prove that he had destroyed a mine-layer, and that his bomb had blown up several mines stored on the craft. This evidence took the form of fragments of mines. "Some of these pieces must even have been driven up against our hull," Darrin declared. "It is a wonder that we were not sunk." "The counter pressure of the water would lessen the force of these fragments, especially after they had been blown out through the shell of the submarine," Lieutenant Fernald argued. "But I agree with you, sir, that it's a wonder the 'Grigsby' suffered nothing worse than a shaking." Other evidence, too, the divers sent up. The destroyed craft had surely been a mine-laying submarine. The divers measured the length of the wrecked hull, finding it to be close to three hundred feet. They reported, too, that scores of German dead lay in the wreckage. For hours nothing more happened. Just before ten o'clock that night the mine-sweeper's blinkers signalled a call to the "Grigsby," then about four miles distant.