"All right. I'm with you." By the time they had reached the alley it was almost entirely dark. Choosing a moment when the street was empty, they slipped into the alley and made their way toward the further end. They felt the walls on either side as they went along for indications of a door or opening of any kind. They did the same with the blank wall that closed the alley at the other end. Nothing rewarded their search. The wall at the farther end was far too high to scale. It seemed impossible for anything except with wings to vanish from the alley as completely as had their assailants on that memorable night when they had so nearly lost their lives. "It beats me," said Bart at length. "We saw them go in and we followed them up and they weren't there. Sounds like black magic, doesn't it?" "It surely does," agreed Frank, in great perplexity. "They didn't go through the back, they couldn't go through the sides, they couldn't go into the air, but they did go somewhere." "Down into the ground," suggested Bart jokingly. "That seems the only place left." Frank started. "There's many a true word spoken in jest," he said. "Perhaps you've hit it, Bart. That's one place we haven't examined." "Small chance to examine that just now," said Bart. "You can see it's all covered with a glare of ice. There isn't a bit of ground showing." They walked over the ice-covered surface with scarcely a hope under present conditions of making any discoveries, even if there were any to make. They had to depend entirely upon the sense of touch, for it was by this time pitch dark and Frank did not care to flash his light for fear they might be observed by passers-by. They had come perhaps to within twenty feet of the rear wall, when Frank gave a sudden exclamation. CHAPTER XI MARSHAL FOCH AND GENERAL PERSHING "What is it, Frank?" asked Bart Raymond in a low voice. "My foot sank in," explained Frank. "It's softer here for some reason than in the rest of the alley. Just wait a minute till I can feel around here and see what I can make of it." He felt about cautiously with his feet and found that the ice had softened for a space of about four feet and that the space was almost perfectly square. "There's some reason why this spot should be different from the rest," he said, after having verified his discovery. "It's all open to the weather, like every other part of the alley, and there's only one explanation. There's heat coming up from beneath. That means that there must be an open space beneath this spot. I wish I dared use my flashlight." "Wait a minute," said Bart. "I'll slip out to the mouth of the alley and see if the coast is clear. If it is, I'll give a low whistle and then you flash your light and see what it shows you." He left his companion, and a moment after Frank heard the signal agreed upon. Instantly he flashed his light on the rectangular space that had caught his attention.