"That's queer!" exclaimed Frank, gazing above the blank wall at the outline of a tall building that rose beyond it. "What's queer?" asked Billy. "Why, that building there is the same one the doctor went into," answered his companion. "I know it by that cupola on the top. It must back up right against this wall. In fact, this wall is part of the rear wall of the house. I thought these were only factories." "Oh, well, what if it is?" returned Bart. "We'd better get busy here before we're interrupted. Let's hope there isn't another fire in this district to-night." Without much difficulty they found the square place that Frank and Bart had noticed on their previous visit. They scraped away the ice and gravel and discovered the ring by which the trap door was evidently raised. Then they braced themselves and gave a mighty tug. But the effort was unavailing. They were far stronger than the ordinary run of men, and yet even their trained muscles had to confess defeat. "Perhaps it's locked or bolted on the other side," suggested Bart. "Not likely," answered Frank. "It's more probable that it's frozen in. Get out your knives and dig around the edge of the door, and then we'll try again." They did this for perhaps five minutes, and then tried again. This time the door moved but did not yield. Once more they bent their backs to the work, and this time they won. Slowly and creakingly the door rose, showing a yawning chasm beneath, while a rush of fetid air assailed their nostrils! CHAPTER XVII A PERILOUS SITUATION The three Army Boys started back almost letting go the trap door in their desire to escape the noxious odor and fill their lungs with the cool winter air. "What is this anyway--the entrance to the infernal regions?" asked Billy. "If it were, it couldn't smell much worse, I imagine," answered Bart. "We're not going to let a thing like that hold us back, are we?" asked Frank impatiently. "Of course not," replied Billy. "But that doesn't say we have to like it, does it? Flash that light of yours and let us see just what this sweet smelling thing looks like." Frank directed the rays of his flashlight into the gloomy recess, and the light fell on a small platform about four feet below the level of the ground. Two or three stone steps descended from this and then they could faintly see a rough stone floor from which several passages branched out in different directions. He returned the light to his pocket, and the three held a whispered conversation. "Well, fellows, you've seen as much of it as I have," said Frank. "What do you say? Shall we explore it?" "Sure thing," replied Bart. "What do you think we are, a bunch of four flushers?" "Lead on, old scout," said Billy. "But first we must wedge this door up a trifle, so as to be able to open it easily when we come back." "Right you are!" said Frank. "When we do come back we may have to come in a hurry for all we know, and we want to be able to lift this up in a jiffy."