Without pausing to consider that it might not be safe to investigate the abandoned manufacturing plant at such a late hour, the girls drove directly into the hilly section of Belton City. Penny turned into the familiar dead-end street and was relieved to find no sign of other vehicles. She halted her roadster at the very end of the pavement in such a position that the bright headlights played upon the massive walls of the Hamilton building. "It must be located higher up," Penny murmured to herself. "What is?" Susan demanded. "I don't see what you're about anyway." Without answering, Penny directed the beam of her spotlight upon the stone structure. Inch by inch she moved it systematically over the high wall. "Perhaps it's only a silly idea," she acknowledged at last, "but I believe that somewhere in the wall there must be a secret door--one mechanically operated. No doubt the outline of the opening is disguised by the many irregular cracks in the masonry." "If you're looking for a secret opening, why not come in the daytime when you can see much better?" "I've been here in the daytime and the door can't be detected--at least not by the eye. I'm hoping to have better luck this time." "I can't for the life of me see how," Susan began, but ended with a startled gasp. A portion of the massive wall was slowly moving backward. "Just as I thought!" Penny chuckled in delight. "Now we know how Rap Molberg escaped from the police the other night." In fascination the girls watched the widening gap in the wall. Soon it was large enough for an automobile to easily drive through into the empty building. "How did you open it?" Susan asked in awe. "The beam of my spotlight struck a photo-electric cell which was secreted near the eaves," Penny explained briefly. "You should have read about it at the library." "I wish I had now. It's almost uncanny." "Let's drive in and have a look at the inside," Penny suggested daringly. "Won't it be dangerous?" Susan demurred. "The place seems to be deserted. But probably it would be wiser if you waited here and I went in alone." "No, if you're going to risk it, so am I!" "Then here goes," Penny said. She drove the roadster through the opening into what appeared to be an empty room. Curiously, the girls glanced about. Suddenly Susan uttered a stifled scream. "The door! It's closing!" Already the opening had narrowed to a mere slit. It was too late to retreat. "Don't lose your nerve," Penny advised, although her own heart was beating at a furious rate. "We'll find some way to open that door." "Someone may have seen us drive in and closed it deliberately!" "I don't think so, Susan. It must have closed automatically." "Anyway, we're prisoners inside this horrible place! We'll starve to death before anyone will suspect we're here!" "I got you into this and I'll get you out," Penny announced firmly. "There must be some button or lever that opens the door from the inside."