Callahan knew that the four principal ports of embarkation for troops--Boston, New York, Norfolk, and Charleston--were shrouded in a mantle of secrecy which it was almost impossible to penetrate. Some months before, when he had been working on the case which grew out of the disappearance of the plans of the battleship _Pennsylvania_, he had had occasion to make a number of guarded inquiries in naval circles in New York, and he recalled that it had been necessary not only to show his badge, but to submit to the most searching scrutiny before he was allowed to see the men he wished to reach. He therefore felt certain that no outsider could have dug up the specific information in the short space of time at their disposal. But, arguing that it had been obtained, the way in which it had been passed on to the U-boat also presented a puzzle. Was there a secret submarine base on the coast? Had some German, more daring than the rest, actually come ashore and penetrated into the very lines of the Service? Had he laid a plan whereby he could repeat this operation as often as necessary? Or did the answer lie in a concealed wireless, operating upon information supplied through underground channels? These were only a few of the questions which raced through Callahan's mind. The submarine base he dismissed as impracticable. He knew that the _Thor_, the _Unita_, the _Macedonia_, and nine other vessels had, at the beginning of the war, cleared from American ports under false papers with the intention of supplying German warships with oil, coal, and food. He also knew that, of the million and a half dollars' worth of supplies, less than one-sixth had ever been transshipped. Therefore, having failed so signally here, the Germans would hardly try the same scheme again. The rumor that German officers had actually come into New York, where they were supposed to have been seen in a theater, was also rather far-fetched. So the wireless theory seemed to be the most tenable. But even a wireless cannot conceal its existence from the other stations indefinitely. Of course, it was possible that it might be located on some unfrequented part of the coast--but then how could the operator obtain the information which he transmitted to the U-boat? Callahan gave it up in despair--for that night. He was tired and he felt that eight hours' sleep would do him more good than thrashing around with a problem for which there appeared to be no solution; a problem which, after all, he couldn't even be sure existed. Maybe, he thought, drowsily, as he turned off the light--maybe the German on the U-boat was only boasting, after all--or, maybe....