"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!